Deduplicate incompatible actual descriptors

Otherwise, we might get false positive AMBIGUOUS_ACTUALS due to one and
the same descriptor appearing multiple times in a list

idea/testData/multiModuleHighlighting/hierarchicalExpectActualMatching/weaklyIncompatibleActualInIntermediateModule/top/top.kt

@@ -1,5 +1,5 @@
 package sample
 
-expect class <!AMBIGUOUS_ACTUALS("Class 'A'", "middle, middle")!>A<!> {
+expect class A {
     fun f()
 }