diff --git a/src/assign.c b/src/assign.c
index ef3542f770..42a4022b27 100644
--- a/src/assign.c
+++ b/src/assign.c
@@ -475,7 +475,7 @@ SEXP assign(SEXP dt, SEXP rows, SEXP cols, SEXP newcolnames, SEXP values)
           Rprintf("RHS for item %d has been duplicated because NAMED==%d MAYBE_SHARED==%d, but then is being plonked. length(values)==%d; length(cols)==%d)\n",
                   i+1, NAMED(thisvalue), MAYBE_SHARED(thisvalue), length(values), length(cols));
         }
-        thisvalue = duplicate(thisvalue);   // PROTECT not needed as assigned as element to protected list below.
+        thisvalue = copyAsPlain(thisvalue);   // PROTECT not needed as assigned as element to protected list below.
       } else {
         if (verbose) Rprintf("Direct plonk of unnamed RHS, no copy. NAMED==%d, MAYBE_SHARED==%d\n", NAMED(thisvalue), MAYBE_SHARED(thisvalue));  // e.g. DT[,a:=as.character(a)] as tested by 754.5
       }
@@ -564,7 +564,7 @@ SEXP assign(SEXP dt, SEXP rows, SEXP cols, SEXP newcolnames, SEXP values)
             if (iRHS[0] != NA_INTEGER) warning("Coerced '%s' RHS to 'integer' to match the factor column's underlying type. Character columns are now recommended (can be in keys), or coerce RHS to integer or character first.", type2char(TYPEOF(thisvalue)));
           } else { // thisvalue is integer
             // make sure to copy thisvalue. May be modified below. See #2984
-            RHS = PROTECT(duplicate(thisvalue)); thisprotecti++;
+            RHS = PROTECT(copyAsPlain(thisvalue)); thisprotecti++;
             iRHS = INTEGER(RHS);
           }
           for (int j=0; j<length(RHS); j++) {
@@ -735,7 +735,7 @@ SEXP assign(SEXP dt, SEXP rows, SEXP cols, SEXP newcolnames, SEXP values)
   if (anytodelete) {
     // Delete any columns assigned NULL (there was a 'continue' earlier in loop above)
     // In reverse order to make repeated memmove easy. Otherwise cols would need to be updated as well after each delete.
-    PROTECT(colorder = duplicate(cols)); protecti++;
+    PROTECT(colorder = copyAsPlain(cols)); protecti++;
     R_isort(INTEGER(colorder),LENGTH(cols));
     PROTECT(colorder = match(cols, colorder, 0)); protecti++;  // actually matches colorder to cols (oddly, arguments are that way around)
     // Can't find a visible R entry point to return ordering of cols, above is only way I could find.
@@ -809,9 +809,9 @@ const char *memrecycle(SEXP target, SEXP where, int start, int len, SEXP source)
     // SEXP pointed to.
     // If source is already not named (because j already created a fresh unnamed vector within a list()) we don't want to
     // duplicate unnecessarily, hence checking for named rather than duplicating always.
-    // See #481, #1270 and tests 1341.* fail without this duplicate().
+    // See #481, #1270 and tests 1341.* fail without this copy.
     if (anyNamed(source)) {
-      source = PROTECT(duplicate(source)); protecti++;
+      source = PROTECT(copyAsPlain(source)); protecti++;
     }
   }
   if (!length(where)) {  // e.g. called from rbindlist with where=R_NilValue
diff --git a/src/coalesce.c b/src/coalesce.c
index 81e8156341..70a0d0999a 100644
--- a/src/coalesce.c
+++ b/src/coalesce.c
@@ -46,7 +46,7 @@ SEXP coalesce(SEXP x, SEXP inplaceArg) {
       error("Item %d is length %d but the first item is length %d. Only singletons are recycled.", i+2, length(item), nrow);
   }
   if (!inplace) {
-    first = PROTECT(duplicate(first)); nprotect++;
+    first = PROTECT(copyAsPlain(first)); nprotect++;
     if (verbose) Rprintf("coalesce copied first item (inplace=FALSE)\n");
   }
   void **valP = (void **)R_alloc(nval, sizeof(void *));
diff --git a/src/data.table.h b/src/data.table.h
index a5579400c4..9592c4d638 100644
--- a/src/data.table.h
+++ b/src/data.table.h
@@ -217,6 +217,7 @@ void coerceFill(SEXP fill, double *dfill, int32_t *ifill, int64_t *i64fill);
 SEXP coerceFillR(SEXP fill);
 bool INHERITS(SEXP x, SEXP char_);
 bool Rinherits(SEXP x, SEXP char_);
+SEXP copyAsPlain(SEXP x);
 void copySharedColumns(SEXP x);
 SEXP lock(SEXP x);
 SEXP unlock(SEXP x);
diff --git a/src/fmelt.c b/src/fmelt.c
index af5e3a825b..0764de03cd 100644
--- a/src/fmelt.c
+++ b/src/fmelt.c
@@ -694,7 +694,7 @@ SEXP fmelt(SEXP DT, SEXP id, SEXP measure, SEXP varfactor, SEXP valfactor, SEXP
   // edge case no measure.vars
   if (!data.lmax) {
     ans = shallowwrapper(DT, data.idcols);
-    ans = PROTECT(duplicate(ans)); protecti++;
+    ans = PROTECT(copyAsPlain(ans)); protecti++;
   } else {
     ansvals = PROTECT(getvaluecols(DT, dtnames, LOGICAL(valfactor)[0], verbose, &data)); protecti++;
     ansvars = PROTECT(getvarcols(DT, dtnames, LOGICAL(varfactor)[0], verbose, &data)); protecti++;
diff --git a/src/reorder.c b/src/reorder.c
index ec3824efa0..661820bb8a 100644
--- a/src/reorder.c
+++ b/src/reorder.c
@@ -19,7 +19,7 @@ SEXP reorder(SEXP x, SEXP order)
         error("Column %d is length %d which differs from length of column 1 (%d). Invalid data.table.", i+1, length(v), nrow);
       if (SIZEOF(v) > maxSize)
         maxSize=SIZEOF(v);
-      if (ALTREP(v)) SET_VECTOR_ELT(x,i,duplicate(v));  // expand compact vector in place ready for reordering by reference
+      if (ALTREP(v)) SET_VECTOR_ELT(x, i, copyAsPlain(v));
     }
     copySharedColumns(x); // otherwise two columns which point to the same vector would be reordered and then re-reordered, issues linked in PR#3768
   } else {
@@ -33,7 +33,7 @@ SEXP reorder(SEXP x, SEXP order)
   if (!isInteger(order)) error("order must be an integer vector");
   if (length(order) != nrow) error("nrow(x)[%d]!=length(order)[%d]",nrow,length(order));
   int nprotect = 0;
-  if (ALTREP(order)) { order=PROTECT(duplicate(order)); nprotect++; }  // TODO: how to fetch range of ALTREP compact vector
+  if (ALTREP(order)) { order=PROTECT(copyAsPlain(order)); nprotect++; }  // TODO: if it's an ALTREP sequence some optimizations are possible rather than expand
 
   const int *restrict idx = INTEGER(order);
   int i=0;
diff --git a/src/subset.c b/src/subset.c
index 7891a36dd5..f9426bb402 100644
--- a/src/subset.c
+++ b/src/subset.c
@@ -281,7 +281,7 @@ SEXP subsetDT(SEXP x, SEXP rows, SEXP cols) {
     for (int i=0; i<LENGTH(cols); i++) {
       SEXP thisCol = VECTOR_ELT(x, colD[i]-1);
       checkCol(thisCol, colD[i], nrow, x);
-      SET_VECTOR_ELT(ans, i, duplicate(thisCol));
+      SET_VECTOR_ELT(ans, i, copyAsPlain(thisCol));
       // materialize the column subset as we have always done for now, until REFCNT is on by default in R (TODO)
     }
   } else {
diff --git a/src/transpose.c b/src/transpose.c
index 567a9a80c7..8ddf96fdff 100644
--- a/src/transpose.c
+++ b/src/transpose.c
@@ -8,7 +8,7 @@ SEXP transpose(SEXP l, SEXP fill, SEXP ignoreArg, SEXP keepNamesArg) {
   if (!isNewList(l))
     error("l must be a list.");
   if (!length(l))
-    return(duplicate(l));
+    return(copyAsPlain(l));
   if (!isLogical(ignoreArg) || LOGICAL(ignoreArg)[0]==NA_LOGICAL)
     error("ignore.empty should be logical TRUE/FALSE.");
   bool ignore = LOGICAL(ignoreArg)[0];
diff --git a/src/utils.c b/src/utils.c
index 4476384b35..c6b0dc52c7 100644
--- a/src/utils.c
+++ b/src/utils.c
@@ -173,36 +173,88 @@ bool Rinherits(SEXP x, SEXP char_) {
  return ans;
 }
 
+SEXP copyAsPlain(SEXP x) {
+  // v1.12.2 and before used standard R duplicate() to do this. But that's not guaranteed to not return an ALTREP.
+  // e.g. ALTREP 'wrapper' on factor column (with materialized INTSXP) in package VIM under example(hotdeck)
+  //      .Internal(inspect(x[[5]]))
+  //      @558adf4d9508 13 INTSXP g0c0 [OBJ,NAM(7),ATT]  wrapper [srt=-2147483648,no_na=0]
+  // 'AsPlain' is intended to convey unALTREP-ing; i.e. materializing and removing any ALTREP attributes too
+  // For non-ALTREP this should do the same as R's duplicate(); but doesn't quite currently, so has to divert to duplicated() for now
+  // Intended for use on columns; to either un-ALTREP them or duplicate shared memory columns; see copySharedColumns() below
+  // Not intended to be called on a DT VECSXP where a concept of 'deep' might refer to whether the columns are copied
+
+  if (!ALTREP(x)) return duplicate(x);
+  // would prefer not to have this line, but without it test 1639.064 fails :
+  //   Running test id 1639.064      Error in `[.data.table`(r, -ii) :
+  //   Item 2 of i is -1 and item 1 is NA. Cannot mix negatives and NA.
+  //   Calls: test.data.table ... FUN -> make.levels -> rbindlist -> [ -> [.data.table
+  // Perhaps related to row names and the copyMostAttrib() below is not quite sufficient
+
+  size_t n = XLENGTH(x);
+  SEXP ans = PROTECT(allocVector(TYPEOF(x), XLENGTH(x)));
+  switch (TYPEOF(ans)) {
+  case RAWSXP:
+    memcpy(RAW(ans),     RAW(x),     n*sizeof(Rbyte));           // # nocov; add coverage when ALTREP is turned on for all types
+    break;                                                       // # nocov
+  case LGLSXP:
+    memcpy(LOGICAL(ans), LOGICAL(x), n*sizeof(Rboolean));        // # nocov
+    break;                                                       // # nocov
+  case INTSXP:
+    memcpy(INTEGER(ans), INTEGER(x), n*sizeof(int));             // covered by 10:1 after test 178
+    break;
+  case REALSXP:
+    memcpy(REAL(ans),    REAL(x),    n*sizeof(double));          // covered by as.Date("2013-01-01")+seq(1,1000,by=10) after test 1075
+    break;
+  case CPLXSXP:
+    memcpy(COMPLEX(ans), COMPLEX(x), n*sizeof(Rcomplex));        // # nocov
+    break;                                                       // # nocov
+  case STRSXP: {
+    const SEXP *xp=STRING_PTR(x);                                // covered by as.character(as.hexmode(1:500)) after test 642
+    for (R_xlen_t i=0; i<n; ++i) SET_STRING_ELT(ans, i, xp[i]);
+  } break;
+  case VECSXP: {
+    const SEXP *xp=VECTOR_PTR(x);                                // # nocov
+    for (R_xlen_t i=0; i<n; ++i) SET_VECTOR_ELT(ans, i, xp[i]);  // # nocov
+  } break;                                                       // # nocov
+  default:
+    error("Internal error: unsupported type '%s' passed to copyAsPlain()", type2char(TYPEOF(x))); // # nocov
+  }
+  copyMostAttrib(x, ans); // e.g. factor levels, class etc, but not names, dim or dimnames
+  if (ALTREP(ans))
+    error("Internal error: type '%s' passed to copyAsPlain() but it seems copyMostAttrib() retains ALTREP attributes", type2char(TYPEOF(x))); // # nocov
+  UNPROTECT(1);
+  return ans;
+}
+
 void copySharedColumns(SEXP x) {
   const int ncol = length(x);
   if (!isNewList(x) || ncol==1) return;
-  bool *shared = (bool *)R_alloc(ncol, sizeof(bool)); // on R heap in case duplicate() fails
+  bool *shared = (bool *)R_alloc(ncol, sizeof(bool)); // on R heap in case alloc fails
   int *savetl = (int *)R_alloc(ncol, sizeof(int));  // on R heap for convenience but could be a calloc
-  int nShared = 0;
+  int nShared=0, thistl=0;
   const SEXP *xp = VECTOR_PTR(x);
   for (int i=0; i<ncol; ++i) {
     SEXP thiscol = xp[i];
-    const int thistl = TRUELENGTH(thiscol);
-    if (thistl<0) {
+    if (ALTREP(thiscol) || (thistl=TRUELENGTH(thiscol))<0) {
       shared[i] = true;
       nShared++;
-      // do not duplicate() here as the duplicate() might fail. Careful to restore tl first to all columns.
+      // do not copyAsPlain() here as the alloc might fail: careful to restore tl first to all columns.
       // Aside: thistl is which column shares the same address as this one in case that's ever useful in future.
     } else {
       shared[i] = false;
       savetl[i] = thistl;  // these are vectors which are all expected to have tl, unlike CHARSXP which often don't (savetl() has CHARSXP in mind)
-      SET_TRUELENGTH(thiscol, -i-1);
+      SET_TRUELENGTH(thiscol, -i-1);  // just on plain vectors, not on ALTREP
     }
   }
   // now we know nShared and which ones they are (if any), restore original tl back to all columns
   for (int i=0; i<ncol; ++i) {
     if (!shared[i]) SET_TRUELENGTH(VECTOR_ELT(x, i), savetl[i]);
   }
-  // now that truelength has been restored for all columns, we can finally call duplicate()
+  // now that truelength has been restored for all columns, we can finally call copyAsPlain()
   if (nShared) {
     for (int i=0; i<ncol; ++i) {
       if (shared[i])
-        SET_VECTOR_ELT(x, i, duplicate(VECTOR_ELT(x, i)));
+        SET_VECTOR_ELT(x, i, copyAsPlain(VECTOR_ELT(x, i)));
     }
     if (GetVerbose()) Rprintf("Found and copied %d column%s with a shared memory address\n", nShared, nShared>1?"s":"");
     // GetVerbose() (slightly expensive call of all options) called here only when needed
diff --git a/src/wrappers.c b/src/wrappers.c
index 2dc98264ac..928be0c05a 100644
--- a/src/wrappers.c
+++ b/src/wrappers.c
@@ -93,7 +93,7 @@ SEXP expandAltRep(SEXP x)
   for (int i=0; i<LENGTH(x); i++) {
     SEXP col = VECTOR_ELT(x,i);
     if (ALTREP(col)) {
-      SET_VECTOR_ELT(x, i, duplicate(col));
+      SET_VECTOR_ELT(x, i, copyAsPlain(col));
     }
   }
   return R_NilValue;