FIX: SAME? on VECTORs return TRUE even when vectors are not identical

fixes: metaeducation/rebol-issues#2400

src/core/t-vector.c

@@ -586,6 +586,9 @@ void Set_Vector_Row(REBSER *ser, REBVAL *blk)
 /*
 ***********************************************************************/
 {
+	if (mode == 3)
+		return VAL_SERIES(a) == VAL_SERIES(b) && VAL_INDEX(a) == VAL_INDEX(b);
+
 	REBINT n = Compare_Vector(a, b);  // needs to be expanded for equality
 	if (mode >= 0) {
 		return n == 0;

src/tests/units/vector-test.r3

@@ -359,6 +359,18 @@ Rebol [
 
 ===start-group=== "VECTOR copy"
 
+--test-- "COPY"
+	;@@ https://github.com/rebol/rebol-issues/issues/2400
+	v1: #[ui16! [1 2]]
+	v2: v1
+	v3: copy v2
+	--assert     same? v1 v2
+	--assert not same? v1 v3
+	v2/1: 3
+	--assert v1/1 = 3
+	--assert v3/1 = 1
+
+
 --test-- "COPY/PART"
 	;@@ https://github.com/rebol/rebol-issues/issues/2399
 	v: #[ui16! [1 2 3 4]]