One way could be to combine your lookup-value with a counter-information but I assume that you don't want really to access a possible n-th value else that it should return all available return-values of a lookup-value. In this case you could just apply an aggregation in the mapping table, like:
m: mapping load LookupValue, concat(ReturnValue, ' & ') from Source group by LookupValue;
About the best I have is a point to the Design Blog area to have a look at the posts there to see if you can find something that may fit your use case there. I am putting the ApplyMap one directly below, but I am providing the base link for you after that where you can do your own searching to see if you can find some other examples on how you might approach things.
There are at least a few hundred posts out here these days, mostly how-to etc., so hopefully this will be a good resource for you. Sorry I am not more help.
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