This used to work, but it's broken in the latest release at least. CombineVariants with the --assumeIdenticalSamples option complains if there are multiple vcfs with duplicate sample names.
Can you post your entire log output with the error? I just tried it, and my error told me to include the --genotypemergeoption. You can read more about this argument here: https://www.broadinstitute.org/gatk/gatkdocs/org_broadinstitute_gatk_tools_walkers_variantutils_CombineVariants.php#--genotypemergeoption
It turns out this bug is fixed in the nightly builds. If you would like to use the nightly build, you can download it here: https://www.broadinstitute.org/gatk/nightly