Partial fix for KT-470: Remove duplicating errors on the same element.

Redeclarations for top-level properties still get two REDECLARATION diagnostics. (In overload resolver and declaration resolver)
Also added test for redeclaring extension properties.
Also remove useless null check.

compiler/testData/diagnostics/tests/redeclarations/PropertyAndFunInClass.kt

@@ -0,0 +1,9 @@
+class A() {
+    val a: Int = 1
+    fun a(): Int = 2
+}
+
+class B() {
+    fun b(): Int = 2
+    val b: Int = 1
+}