DexArchiveMergerException: Unable to merge dex - Android Studio 3.0 Stable

Add

implementation('commons-validator:commons-validator:1.4.1') {
        exclude group: 'commons-collections', module: 'commons-collections'
    }

instead of

implementation'commons-validator:commons-validator:1.4.1'

The reason I've tried to explain here if you want to understand the issue and solution.

Hope it solves your issue.