What we are talking about if we use the cola analogy would be 2...

What we are talking about if we use the cola analogy would be 2 different companies using the trademarked name coke as their product brand. It would never happen as coca cola would shut down any other company that tried to use their trademark brand. This is what i dont understand why the company who owns the trademark would allow another company to use it. Especially as both products maybe side by side on a pharmacy shelve.