Calling llListen() in different events...

If reference to:

/54/c0/185465/1.html

After a state_entry, on_rez, or attach to get the current llGetOwner key. Would calling llListen with that new found key be harmful if llListen is called in multiple events?

To be safe, you could use llListenRemove() before calling a new listen (if you only want to open one listen at a time).

For example, declare a global integer:
integer handle;

Then, when you open a listen, remove any existing listen first:
llListenRemove(handle);
handle = llListen(channel, "", llGetOwner(), "";