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/ConflictingExtensionProperties.kt

@@ -0,0 +1,4 @@
+package foo
+
+val Int.<!REDECLARATION!>foo<!> = 2
+val Int.<!REDECLARATION!>foo<!> = 3

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
+}

compiler/testData/diagnostics/tests/redeclarations/kt470.kt

@@ -0,0 +1,37 @@
+val <!REDECLARATION, REDECLARATION!>a<!> : Int = 1
+val <!REDECLARATION, REDECLARATION!>a<!> : Int = 1
+val <!REDECLARATION, REDECLARATION!>a<!> : Int = 1
+
+val <!REDECLARATION, REDECLARATION!>b<!> : Int = 1
+val <!REDECLARATION, REDECLARATION!>b<!> : Int = 1
+val <!REDECLARATION, REDECLARATION!>b<!> : Int = 1
+val <!REDECLARATION, REDECLARATION!>b<!> : Int = 1
+
+<!CONFLICTING_OVERLOADS!>fun foo()<!> {} // and here too
+<!CONFLICTING_OVERLOADS!>fun foo()<!> {} // and here
+<!CONFLICTING_OVERLOADS!>fun foo()<!> {} // and here
+<!CONFLICTING_OVERLOADS!>fun foo()<!> {} // and here
+
+<!CONFLICTING_OVERLOADS!>fun bar()<!> {} // and here
+<!CONFLICTING_OVERLOADS!>fun bar()<!> {} // and here
+<!CONFLICTING_OVERLOADS!>fun bar()<!> {} // and here
+
+class A {
+    val <!REDECLARATION!>a<!> : Int = 1
+    val <!REDECLARATION!>a<!> : Int = 1
+    val <!REDECLARATION!>a<!> : Int = 1
+
+    val <!REDECLARATION!>b<!> : Int = 1
+    val <!REDECLARATION!>b<!> : Int = 1
+    val <!REDECLARATION!>b<!> : Int = 1
+    val <!REDECLARATION!>b<!> : Int = 1
+
+    <!CONFLICTING_OVERLOADS!>fun foo()<!> {} // and here too
+    <!CONFLICTING_OVERLOADS!>fun foo()<!> {} // and here
+    <!CONFLICTING_OVERLOADS!>fun foo()<!> {} // and here
+    <!CONFLICTING_OVERLOADS!>fun foo()<!> {} // and here
+
+    <!CONFLICTING_OVERLOADS!>fun bar()<!> {} // and here
+    <!CONFLICTING_OVERLOADS!>fun bar()<!> {} // and here
+    <!CONFLICTING_OVERLOADS!>fun bar()<!> {} // and here
+}