[K2] Avoid shortening duplicated PSI elements

The existing K2 reference shortener collects all the PSI elements to
shorten. As a result, it possibly shortens duplicated PSI elements. For
example,
```
// FILE: main.kt
package a.b.c

fun test(n: Int) {
    return if (<expr>x.y.z.Outer.Inner.VALUE0 > x.y.z.Outer.Inner.VALUE1</expr>) 1
    else n
}
// FILE: values.kt
package x.y.z

class Outer {
    object Inner {
        val VALUE0 = 13
        val VALUE1 = 17
    }
}
```
for the above code, the existing K2 reference shortener tried to shorten
- x.y.z.Outer.Inner -> Inner
- x.y.z.Outer.Inner.VALUE0 -> VALUE0
- x.y.z.Outer.Inner -> Inner
- x.y.z.Outer.Inner.VALUE1 -> VALUE1

`x.y.z.Outer.Inner` is included in the list to shorten twice.
When it actually shortens the PSI elements, it shortens only
- x.y.z.Outer.Inner.VALUE0 -> VALUE0
- x.y.z.Outer.Inner.VALUE1 -> VALUE1

but it imports all of
- x.y.z.Outer.Inner
- x.y.z.Outer.Inner.VALUE0
- x.y.z.Outer.Inner.VALUE1

As a result, it has unnecessary additional import directives.
This commit fixes the issue by avoiding duplicated shortening for a
single PSI element.

analysis/analysis-api-standalone/tests-gen/org/jetbrains/kotlin/analysis/api/standalone/fir/test/cases/generated/cases/references/FirStandaloneNormalAnalysisSourceModuleReferenceShortenerTestGenerated.java

@@ -184,6 +184,12 @@ public class FirStandaloneNormalAnalysisSourceModuleReferenceShortenerTestGenera
         runTest("analysis/analysis-api/testData/components/referenceShortener/referenceShortener/enumEntryInitUsesCompanion2.kt");
     }
 
+    @Test
+    @TestMetadata("multipleImport.kt")
+    public void testMultipleImport() throws Exception {
+        runTest("analysis/analysis-api/testData/components/referenceShortener/referenceShortener/multipleImport.kt");
+    }
+
     @Test
     @TestMetadata("nestedClass.kt")
     public void testNestedClass() throws Exception {