import java.util.stream.Collectors
import java.util.stream.Stream
import groovy.transform.Field

@Field
Map<String, String> sampleMap = [
        'SampleKey1': 'SampleValue1',
        'SampleKey2': 'SampleValue2'
]

//example param value: "C/SampleKey1/someFile, C/SampleKey2/someFile2"
private Collection<String> getValues(String param) {
    Stream.of(param.split(','))
            .map { getValueFromOnePath(it) }
            .filter { !it.isEmpty() }
            .distinct()
            .collect(Collectors.toList())
}

private String getValueFromOnePath(String path) {
    String[] pathParts = path.split('/')
    if (pathParts.size() < 2) {
        return ''
    }
    return sampleMap[pathParts[1]] ?: ''
}

"C/SampleKey1/someFile, C/SampleKey2/someFile2"

[SampleValue1, SampleValue2]