The basic idea is to allow merging of key values given a set of strategies, for example:

• first: duplicate keys in the first resource read are retained, later keys are discarded
• latest: the opposite of first.
• append: matching key values are concatenated using a separator (configurable)

An example of such transformer can be seen at https://github.com/griffon/griffon/blob/development/subprojects/gradle-griffon-plugin/src/main/groovy/org/codehaus/griffon/gradle/shadow/transformers/PropertiesFileTransformer.groovy