Repeated bytes being sent through UART

Hi,

I am facing a wierd problem when using the UART.

Each 500ms I send the following packet in the UART:

['C',0x01, 0x04, 't', 'e', 's', 't']

 

Randomly, it sends two times the first caracter:

['C','C',0x01, 0x04, 't', 'e', 's', 't']

 

What may be causing this?