(testsuite) Fix `assert_complete_partial()'

Make sure items are unique before assembling list of completions.

test/lib/library.exp

@@ -256,6 +256,8 @@ proc assert_complete_partial {expected cmd {partial ""} {test ""} {prompt /@} {s
         unresolved "$test"
     } else {
         set pick {}
+        # Make sure expected items are unique
+        set expected [lsort -unique $expected]
         foreach item $expected {
             if {$partial == ""} {set partial [string range $item 0 0]}
                 # Only append item if starting with $partial