Various arguments for container-build are done as nargs='+' or nargs='*' which means they are meant to be:

 --arg1 A B --arg2 A B

But if you think that this works like most other CLI's (including koji-container-build) and repeated arguments are accumulated, you are confused since:

 --arg1=A --arg1=B

Just gives you B. One of the following should be done:

• accumulate all arguments together (does store=append nargs=+ does this? haven't tested)
• error out if one of these arguments is specified multiple times.