Allow multiple identical exported numeric constants (#1341)

This still doesn't allow identical exported label constants.
That can be addressed when or if it's requested for a real use case.

Symbols only store one source filename + line number, so this
arbitrarily keeps the last read symbol as the reported one.

test/link/test.sh

@@ -190,6 +190,15 @@ tryDiff "$test"/out.err "$outtemp"
 tryCmp "$test"/out.gb "$gbtemp"
 evaluateTest
 
+test="same-consts"
+startTest
+"$RGBASM" -o "$otemp" "$test"/a.asm
+"$RGBASM" -o "$gbtemp2" "$test"/b.asm
+continueTest
+rgblinkQuiet -o "$gbtemp" "$gbtemp2" "$otemp" 2>"$outtemp"
+tryDiff "$test"/out.err "$outtemp"
+evaluateTest
+
 test="scramble-romx"
 startTest
 "$RGBASM" -o "$otemp" "$test"/a.asm