epg: avoid repeated attempts to queue the same recording due to

different timers yielding the same match.

We could avoid warnings about ongoing recordings entirely by checking
the filename instead of the tuple (channel, program), but this way we
can see how often the same program is aired more than once within the
same week, which is always nice and shows how crappy TV is nowadays.

Signed-off-by: Javier Marcet <javier@marcet.info>