diff --git a/compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/DeclarationTable.kt b/compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/DeclarationTable.kt
index 7432ffc2b51..2d4da384146 100644
--- a/compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/DeclarationTable.kt
+++ b/compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/DeclarationTable.kt
@@ -20,10 +20,21 @@ class DescriptorTable {
     fun get(descriptor: DeclarationDescriptor) = descriptors[descriptor]
 }
 
-// TODO: We don't manage id clashes anyhow now.
-abstract class GlobalDeclarationTable(private val mangler: KotlinMangler) {
+interface UniqIdClashTracker {
+    fun commit(declaration: IrDeclaration, uniqId: UniqId)
+
+    companion object {
+        val DEFAULT_TRACKER = object : UniqIdClashTracker {
+            override fun commit(declaration: IrDeclaration, uniqId: UniqId) {}
+        }
+    }
+}
+
+abstract class GlobalDeclarationTable(private val mangler: KotlinMangler, private val clashTracker: UniqIdClashTracker) {
     private val table = mutableMapOf<IrDeclaration, UniqId>()
 
+    constructor(mangler: KotlinMangler) : this(mangler, UniqIdClashTracker.DEFAULT_TRACKER)
+
     protected open fun loadKnownBuiltins(builtIns: IrBuiltIns, startIndex: Long): Long {
         var index = startIndex
         builtIns.knownBuiltins.forEach {
@@ -34,7 +45,7 @@ abstract class GlobalDeclarationTable(private val mangler: KotlinMangler) {
 
     open fun computeUniqIdByDeclaration(declaration: IrDeclaration): UniqId {
         return table.getOrPut(declaration) {
-            UniqId(mangler.hashedMangleImpl(declaration), false)
+            UniqId(mangler.hashedMangleImpl(declaration), false).also { clashTracker.commit(declaration, it) }
         }
     }
 
diff --git a/compiler/ir/serialization.js/src/org/jetbrains/kotlin/ir/backend/js/lower/serialization/ir/JsDeclarationTable.kt b/compiler/ir/serialization.js/src/org/jetbrains/kotlin/ir/backend/js/lower/serialization/ir/JsDeclarationTable.kt
index ec2673d578b..324d9dd4ae7 100644
--- a/compiler/ir/serialization.js/src/org/jetbrains/kotlin/ir/backend/js/lower/serialization/ir/JsDeclarationTable.kt
+++ b/compiler/ir/serialization.js/src/org/jetbrains/kotlin/ir/backend/js/lower/serialization/ir/JsDeclarationTable.kt
@@ -8,13 +8,30 @@ package org.jetbrains.kotlin.ir.backend.js.lower.serialization.ir
 import org.jetbrains.kotlin.backend.common.serialization.DeclarationTable
 import org.jetbrains.kotlin.backend.common.serialization.DescriptorTable
 import org.jetbrains.kotlin.ir.descriptors.IrBuiltIns
+import org.jetbrains.kotlin.ir.util.render
 
-class JsDeclarationTable(builtIns: IrBuiltIns, descriptorTable: DescriptorTable) : DeclarationTable(builtIns, descriptorTable, JsMangler) {
+class JsUniqIdClashTracker: UniqIdClashTracker {
+    private val commitedUniqIds = mutableMapOf<UniqId, IrDeclaration>()
 
-    override var currentIndex = PUBLIC_LOCAL_UNIQ_ID_EDGE
+    override fun commit(declaration: IrDeclaration, uniqId: UniqId) {
+        if (uniqId.isLocal) return // don't track local ids
 
-    init {
-        loadKnownBuiltins()
-        currentIndex = 0
+        if (uniqId in commitedUniqIds) {
+            val clashedDeclaration = commitedUniqIds[uniqId]!!
+            // TODO: handle clashes properly
+            error("UniqId clash: $uniqId; Existed declaration ${clashedDeclaration.render()} clashed with new ${declaration.render()}")
+        }
+
+        commitedUniqIds[uniqId] = declaration
+    }
+}
+
+class JsGlobalDeclarationTable(builtIns: IrBuiltIns) : GlobalDeclarationTable(JsMangler, JsUniqIdClashTracker()) {
+    private val FUNCTION_INDEX_START: Long = loadKnownBuiltins(builtIns, PUBLIC_LOCAL_UNIQ_ID_EDGE)
+
+    override fun computeUniqIdByDeclaration(declaration: IrDeclaration): UniqId {
+        return if (isBuiltInFunction(declaration)) {
+            UniqId(FUNCTION_INDEX_START + builtInFunctionId(declaration), false)
+        } else super.computeUniqIdByDeclaration(declaration)
     }
 }
\ No newline at end of file