internal_link: Allow duplicates of the same is operand.

This matches the behavior of the webapp. Such narrow links are
semantically valid and equivalent to the deduplicated link.

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/model/internal_link.dart

@@ -181,9 +181,8 @@ Narrow? _interpretNarrowSegments(List<String> segments, PerAccountStore store) {
         dmElement = ApiNarrowDm(dmIds, negated: negated);
 
       case _NarrowOperator.is_:
-        final isElementOperand = IsOperand.fromRawString(operand);
-        if (isElementOperands.contains(isElementOperand)) return null;
-        isElementOperands.add(isElementOperand);
+        // It is fine to have duplicates of the same [IsOperand].
+        isElementOperands.add(IsOperand.fromRawString(operand));
 
       case _NarrowOperator.near: // TODO(#82): support for near
       case _NarrowOperator.with_: // TODO(#683): support for with

test/model/internal_link_test.dart

@@ -229,6 +229,7 @@ void main() {
       for (final operand in IsOperand.values) {
         List<(String, Narrow?)> sharedCases(Narrow? narrow) => [
             ('/#narrow/is/$operand',                                     narrow),
+            ('/#narrow/is/$operand/is/$operand',                         narrow),
             ('/#narrow/is/$operand/near/1',                              narrow),
             ('/#narrow/is/$operand/with/2',                              narrow),
             ('/#narrow/channel/7-test-here/is/$operand',                 null),