Hi;
In the case where you are executing interrupt routine and again the same interrupt changes level the interrupt shall not be ignored.After you ececute RETI the interrupt logic shall again branch you to 0x3h,vector address of INT0.ALthough in interrupt sources like keybaord IC 8279 this is unlikely to happen as the IC shall keep interrupt high untill you acknowledge it.This
can however happen in sources which generate interrupts but do not bother to wait for ack.
Also it suggests that you might be using a wrong feature of 8051.T0/T1 may be more appropriate
Pranav