diff --git a/conservancy_beancount/reports/query.py b/conservancy_beancount/reports/query.py
index 75881069cd22aaf1fdd3105b9a39a32f03166cc5..086ac09b019ba7a00c3e3cdee0a3511222403d87 100644
--- a/conservancy_beancount/reports/query.py
+++ b/conservancy_beancount/reports/query.py
@@ -157,16 +157,14 @@ class PostingContext:
 class MetaDocs(bc_query_env.AnyMeta):
     """Return a list of document links from metadata."""
     def __init__(self, operands: List[bc_query_compile.EvalNode]) -> None:
-        super(bc_query_env.AnyMeta, self).__init__(operands, list)
+        super(bc_query_env.AnyMeta, self).__init__(operands, set)
         # The second argument is our return type.
         # It should match the annotated return type of __call__.
 
-    def __call__(self, context: PostingContext) -> List[str]:
+    def __call__(self, context: PostingContext) -> Set[str]:
         raw_value = super().__call__(context)
-        if isinstance(raw_value, str):
-            return raw_value.split()
-        else:
-            return []
+        seq = raw_value.split() if isinstance(raw_value, str) else ''
+        return set(seq)
 
 
 class RTField(NamedTuple):
@@ -247,7 +245,7 @@ class RTTicket(bc_query_compile.EvalFunction):
             self._meta_key(meta_op.value)
         if not rest:
             operands.append(bc_query_compile.EvalConstant(sys.maxsize))
-        super().__init__(operands, list)
+        super().__init__(operands, set)
 
     def _rt_key(self, key: str) -> RTField:
         try:
@@ -261,7 +259,7 @@ class RTTicket(bc_query_compile.EvalFunction):
         else:
             raise ValueError(f"metadata key {key!r} does not contain documentation links")
 
-    def __call__(self, context: PostingContext) -> list:
+    def __call__(self, context: PostingContext) -> Set[object]:
         rt_key: str
         meta_key: str
         limit: int
@@ -283,7 +281,7 @@ class RTTicket(bc_query_compile.EvalFunction):
                 ticket_ids.add(rt_id[0])
                 if len(ticket_ids) >= limit:
                     break
-        retval: List[object] = []
+        retval: Set[object] = set()
         for ticket_id in ticket_ids:
             try:
                 rt_ticket = self._rt_cache[ticket_id]
@@ -294,9 +292,9 @@ class RTTicket(bc_query_compile.EvalFunction):
             if field_value is None:
                 pass
             elif isinstance(field_value, list):
-                retval.extend(field_value)
+                retval.update(field_value)
             else:
-                retval.append(field_value)
+                retval.add(field_value)
         return retval