Unnecessary variable: do not suggest in case of name duplication

So #KT-20300 Fixed

idea/src/org/jetbrains/kotlin/idea/inspections/UnnecessaryVariableInspection.kt

@@ -27,12 +27,13 @@ import com.intellij.psi.search.searches.ReferencesSearch
 import org.jetbrains.kotlin.descriptors.FunctionDescriptor
 import org.jetbrains.kotlin.descriptors.VariableDescriptor
 import org.jetbrains.kotlin.idea.caches.resolve.analyze
+import org.jetbrains.kotlin.idea.core.NewDeclarationNameValidator
 import org.jetbrains.kotlin.idea.refactoring.inline.KotlinInlineValHandler
 import org.jetbrains.kotlin.psi.*
-import org.jetbrains.kotlin.psi.psiUtil.getNextSiblingIgnoringWhitespaceAndComments
-import org.jetbrains.kotlin.psi.psiUtil.getParentOfType
+import org.jetbrains.kotlin.psi.psiUtil.*
 import org.jetbrains.kotlin.resolve.BindingContext.DECLARATION_TO_DESCRIPTOR
 import org.jetbrains.kotlin.resolve.BindingContext.REFERENCE_TARGET
+import org.jetbrains.kotlin.resolve.DescriptorToSourceUtils
 
 class UnnecessaryVariableInspection : AbstractKotlinInspection() {
 
@@ -73,7 +74,23 @@ class UnnecessaryVariableInspection : AbstractKotlinInspection() {
                     val initializerDescriptor = context[REFERENCE_TARGET, initializer] as? VariableDescriptor ?: return false
                     if (initializerDescriptor.isVar) return false
                     if (initializerDescriptor.containingDeclaration !is FunctionDescriptor) return false
-                    return ReferencesSearch.search(property, LocalSearchScope(enclosingElement)).findFirst() != null
+
+                    val copyName = initializerDescriptor.name.asString()
+                    if (ReferencesSearch.search(property, LocalSearchScope(enclosingElement)).findFirst() == null) return false
+
+                    val containingDeclaration = property.getStrictParentOfType<KtDeclaration>()
+                    if (containingDeclaration != null) {
+                        val validator = NewDeclarationNameValidator(
+                                container = containingDeclaration,
+                                anchor = property,
+                                target = NewDeclarationNameValidator.Target.VARIABLES,
+                                excludedDeclarations = listOfNotNull(
+                                        DescriptorToSourceUtils.descriptorToDeclaration(initializerDescriptor) as? KtDeclaration
+                                )
+                        )
+                        if (!validator(copyName)) return false
+                    }
+                    return true
                 }
                 return false
             }

idea/testData/inspectionsLocal/unnecessaryVariable/it.kt

@@ -0,0 +1,11 @@
+// PROBLEM: none
+// WITH_RUNTIME
+
+fun foo(a: List<String>, b: List<Int>) {
+    a.forEach {
+        val <caret>a2 = it
+        b.forEach {
+            println(a2.length)
+        }
+    }
+}

idea/testData/inspectionsLocal/unnecessaryVariable/parameterWithSameNameBelow.kt

@@ -0,0 +1,15 @@
+// PROBLEM: none
+
+interface Callback {
+    fun on(id: Int)
+}
+
+fun called(id: Int) {
+    val <caret>calledId = id
+    object : Callback {
+        override fun on(id: Int) {
+            if (id == calledId) {
+            }
+        }
+    }
+}

idea/tests/org/jetbrains/kotlin/idea/inspections/LocalInspectionTestGenerated.java

@@ -2510,12 +2510,24 @@ public class LocalInspectionTestGenerated extends AbstractLocalInspectionTest {
             doTest(fileName);
         }
 
+        @TestMetadata("it.kt")
+        public void testIt() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("idea/testData/inspectionsLocal/unnecessaryVariable/it.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("paramCopy.kt")
         public void testParamCopy() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("idea/testData/inspectionsLocal/unnecessaryVariable/paramCopy.kt");
             doTest(fileName);
         }
 
+        @TestMetadata("parameterWithSameNameBelow.kt")
+        public void testParameterWithSameNameBelow() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("idea/testData/inspectionsLocal/unnecessaryVariable/parameterWithSameNameBelow.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("propertyCopy.kt")
         public void testPropertyCopy() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("idea/testData/inspectionsLocal/unnecessaryVariable/propertyCopy.kt");