Catch multiple records when doing Select Into

I have a SELECT INTO varname columname FROM ... statement and I would
like to raise an exception when that returns more than one record.
GET DIAGNOSTICS rowcount = ROW_COUNT seems to return always one for
this. Currently I find no other way to do that than to run this
statement twice - first to see how many records it returns, which seems
very inefficient.

I can see that this will be handled for 8.2, but for 8.1 what is the
best practice to do this?

Thanks.
SWK