Concatenation and joining are two different operations. You can concatenate tables but indeed they must have different variable names, otherwise it's ambiguous what should be done with the duplicate variables. If you want them to be distinct variables in the result, then you do need a concatenation with [] but you'd have to rename one of the duplicate variable name.
If you are looking at joining the table, then there are different types of joins: left outer join, right outer join, full outer join, inner join, and plain join. All merge the rows of the table which have identical key but they all behave differently with regards to rows that don't match or when there are several match. At the moment, we don't have enough information on what you want to tell you which to use and how to use it.
