In the interrupt service routine for a specific interrupt source, the corresponding bit of the SRCPND register has to be cleared to get the interrupt request from the same ource correctly. If you return from the ISR without clearing the bit, the interrupt controller operates as if another interrupt request came in from the same source. In other words, if a specific bit of the SRCPND register is set to 1, it is always considered as a valid interrupt request waiting to be serviced.
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