Duplicate Id for particle-wall collision.

Hello to everyone.
To identify the particles that collide with the wall, I used this command in python:
ID particle=deck.timestep[t].collision.surfGeom.getFirstIds()

There are many duplicate IDs in ID particle , which confuse me.
Since it is in a specified time step, I anticipate that each particle will only collide with the wall once.
I use this command to find the deposition  and this issue has an impact on my code's output.
Could you please guide me in this field?

Regards,
Fatemeh