Validate that dict keys are unique during parse

src/preserves/private/parse.nim

@@ -54,8 +54,12 @@ proc parsePreserves*(text: string): Preserve[void] {.gcsafe.} =
       var prs = Value(kind: pkDictionary)
       for i in countDown(stack.high.pred, 0, 2):
         if stack[i].pos < capture[0].si: break
-        prs[move stack[i].value] = stack[i.succ].value
-      stack.shrink prs.dict.len*2
+        var
+          val = stack.pop.value
+          key = stack.pop.value
+        for j in 0..prs.dict.high:
+          validate(prs.dict[j].key != key)
+        prs[key] = val
       pushStack prs
 
     Preserves.Set <- Preserves.Set: