diff --git a/compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/KotlinIrLinker.kt b/compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/KotlinIrLinker.kt
index fcfa9adc44e..e61c3756f9c 100644
--- a/compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/KotlinIrLinker.kt
+++ b/compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/KotlinIrLinker.kt
@@ -243,7 +243,7 @@ abstract class KotlinIrLinker(
 
             fileProto.declarationIdList.forEach {
                 val uniqIdKey = it.uniqIdKey(moduleDescriptor)
-                reversedFileIndex.put(uniqIdKey, file)
+                reversedFileIndex.getOrPut(uniqIdKey) { mutableListOf() }.add(file)
             }
 
             when (deserializationStrategy) {
@@ -299,16 +299,16 @@ abstract class KotlinIrLinker(
             return codedInputStream
         }
 
-    private val reversedFileIndex = mutableMapOf<UniqIdKey, IrFile>()
+    private val reversedFileIndex = mutableMapOf<UniqIdKey, MutableList<IrFile>>()
 
     private val UniqIdKey.moduleOfOrigin
         get() =
-            this.moduleDescriptor ?: reversedFileIndex[this]?.packageFragmentDescriptor?.containingDeclaration
+            this.moduleDescriptor ?: reversedFileIndex[this]?.handleClashes(this)?.packageFragmentDescriptor?.containingDeclaration
 
     private fun deserializeTopLevelDeclaration(uniqIdKey: UniqIdKey): IrDeclaration {
         val proto = loadTopLevelDeclarationProto(uniqIdKey)
         return deserializersForModules[uniqIdKey.moduleOfOrigin]!!
-            .deserializeDeclaration(proto, reversedFileIndex[uniqIdKey]!!)
+            .deserializeDeclaration(proto, reversedFileIndex[uniqIdKey]!!.handleClashes(uniqIdKey))
     }
 
     protected abstract fun reader(moduleDescriptor: ModuleDescriptor, uniqId: UniqId): ByteArray
@@ -316,6 +316,14 @@ abstract class KotlinIrLinker(
     protected abstract fun readType(moduleDescriptor: ModuleDescriptor, typeIndex: Int): ByteArray
     protected abstract fun readString(moduleDescriptor: ModuleDescriptor, stringIndex: Int): ByteArray
 
+    protected open fun List<IrFile>.handleClashes(uniqIdKey: UniqIdKey): IrFile {
+        if (size == 1)
+            return this[0]
+        assert(size != 0)
+        error("UniqId clash: ${uniqIdKey.uniqId.index}. Found in the " +
+                      "[${this.joinToString { it.packageFragmentDescriptor.containingDeclaration.name.asString() }}]")
+    }
+
     private fun loadTopLevelDeclarationProto(uniqIdKey: UniqIdKey): KotlinIr.IrDeclaration {
         val stream = reader(uniqIdKey.moduleOfOrigin!!, uniqIdKey.uniqId).codedInputStream
         return KotlinIr.IrDeclaration.parseFrom(stream, newInstance())
@@ -358,7 +366,7 @@ abstract class KotlinIrLinker(
             }
 
             val reachable = deserializeTopLevelDeclaration(key)
-            val file = reversedFileIndex[key]!!
+            val file = reversedFileIndex[key]!!.handleClashes(key)
             file.declarations.add(reachable)
             deserializeFileAnnotationsIfFirstUse(moduleOfOrigin, file)
 
diff --git a/compiler/ir/serialization.js/src/org/jetbrains/kotlin/ir/backend/js/lower/serialization/ir/JsIrLinker.kt b/compiler/ir/serialization.js/src/org/jetbrains/kotlin/ir/backend/js/lower/serialization/ir/JsIrLinker.kt
index a252cdde0a9..d21d335bc22 100644
--- a/compiler/ir/serialization.js/src/org/jetbrains/kotlin/ir/backend/js/lower/serialization/ir/JsIrLinker.kt
+++ b/compiler/ir/serialization.js/src/org/jetbrains/kotlin/ir/backend/js/lower/serialization/ir/JsIrLinker.kt
@@ -9,6 +9,7 @@ import org.jetbrains.kotlin.backend.common.LoggingContext
 import org.jetbrains.kotlin.backend.common.serialization.*
 import org.jetbrains.kotlin.descriptors.ModuleDescriptor
 import org.jetbrains.kotlin.ir.backend.js.kotlinLibrary
+import org.jetbrains.kotlin.ir.declarations.IrFile
 import org.jetbrains.kotlin.ir.descriptors.IrBuiltIns
 import org.jetbrains.kotlin.ir.symbols.IrClassifierSymbol
 import org.jetbrains.kotlin.ir.util.SymbolTable
@@ -45,6 +46,16 @@ class JsIrLinker(
     override fun readString(moduleDescriptor: ModuleDescriptor, stringIndex: Int) =
         moduleDescriptor.kotlinLibrary.string(stringIndex)
 
+    override fun List<IrFile>.handleClashes(uniqIdKey: UniqIdKey): IrFile {
+        if (size == 1)
+            return this[0]
+        assert(size != 0)
+        error("UniqId clash: ${uniqIdKey.uniqId.index}. Found in the " +
+                      "[${this.joinToString { it.packageFragmentDescriptor.containingDeclaration.userName }}]")
+    }
+
+    private val ModuleDescriptor.userName get() = kotlinLibrary.libraryFile.absolutePath
+
     override fun declareForwardDeclarations() {
         // since for `knownBuiltIns` such as FunctionN it is possible to have unbound symbols after deserialization
         // reference them through out lazy symbol table