Frame CheckSum Calculation Problem

Good Morning!

Currently i'm dealing with a Point to Point Protocol through Gsm Connection to access remote system equipments.

Recently i've detected a faulty situation regarding the use of FCS calculation. I'm using rfc code to achieve that objective.

Sometimes, when i'm sending commands to the remote system, there's no kind of response. Through a more patient analysis, i saw that when a packet is like this one:

0xFF, 0x03, 0x40, 0x41, 0x00, 0x00, 0x00, 0x01, 0x00, 0x10, 0x0B, 0x0B

The FCS isn't correct -> Note that the last two bytes are equal and the FCS calculator doesn't consider the last one.

Anyone could help me with this in order to solve it?

Best regards.