Android project: 1.0.0RC - Need gradle 6.1.1, does not work with 5.6.4

Since upgrade from 0.20.0 to 1.0.0-RC along with Kotlin to 1.4.0 the application fails to build with multiple errors in ::sample:checkDebugDuplicateClasses task with messages like:

java.lang.RuntimeException: 
Duplicate class kotlinx.serialization.AbstractSerialFormat found in modules 
jetified-kotlinx-serialization-core-jvm-1.0.0-RC.jar (org.jetbrains.kotlinx:kotlinx-serialization-core-jvm:1.0.0-RC) and 
jetified-kotlinx-serialization-core-jvm-1.0.0-RC.jar (org.jetbrains.kotlinx:kotlinx-serialization-core:1.0.0-RC)

It seems the problem is actual with gradle 5.6.4 and a workaround by upgrading to 6.1.1 fixes the problem. However this bump of major gradle version may not be that easy in many cases. Is it intended or is it a bug? The need for gradle upgrade is not mentioned in migration guide explicitly.

To Reproduce A sample application using the library with kotlinx.serialization dependency.

1. include the library to see build failure. Commit
2. Update gradle to 6.1.1 - build passes. Commit

The library using serialization could be found in the same repository. Here is how the dependency is configured.

Environment

• Kotlin version: 1.4.0
• Library version: 1.0.0-RC
• Kotlin platforms: JVM (Android)
• Gradle version: 5.6.4/6.1.1
• AGP 3.6.3

[pdvrieze]

For 1.0 release the maven coordinates were updated (along with many other parts of the api), so runtime no longer exists as a coordinate, but now is -core. -json provides the json format, but pulls in core transitively

[vorobeij]

I have this issue with Gradle 6.5 and 6.6.1