Buffer IR trigger signal to relay, to supress extra signals
One solution to your problem here is going to be to change the software in that 8051 derivative microcontroller. Another solution is to select another IR remote that uses a different protocol and see if the embedded software in the MCU is smart enough to filter out repeats. 

Virtually all IR remote controls will transmit their code repeatedly when a button is pressed. The typical rate is about 10 times per second. Some types of controls use a simple protocol where every one of the repeated transmissions are the same. Another type will send out the fully encoded transmission for the button press once and then send a shorter sequence than means repeat repeat. Then there are remotes that use Philips RC-5 protocol where there is a "toggle bit" in the protocol that changes state only when a new button press occurs. As long as the receiver sees the same toggle bit value received it knows that it is a repeated transmission.

If you are able to modify the MCU software by replacing the MCU with one that you can re-program then the easiest thing to do is to detect when the arriving transmissions are still repeating at the approximate 10 Hz rate and ignore anything that keeps coming till maybe there is no detection for say half a second.