Looking at the slf4j-jdk14 and slf4j-nop artifacts, they both appear to contain the package `org.slf4j.impl`. Jigsaw will refuse to load two modules that contain the same package, so this will be a problem.
I know that SLF4J does not intend users to load both of these modules at the same time. But the current setup means that it will be the JPMS runtime that rejects it, meaning that there is no chance for SLF4J to output a helpful message (as I believe it does today).
The solution to this would appear to be to move the `org.slf4j.impl` package to `org.slf4j.jul.impl` and `org.slf4j.nop.impl`. As the impl package is not exported, this should not affect any user code (except code that would have been affected anyway).