Received a reminder of already returned KPN equipment

In May,  I canceled my subscription with  KPN.  Last week I received the carton box in order to return KPN equipment.  On Saturday 11 June,  I packet the modem,  TV receiver,  TV receiver remote  and both chargers and mailed the box back to KPN via Post NL. Yesterday I received another email from KPN, informing me that they received the modem but not the TV receiver and urging me to also return the receiver.

It is impossible that KPN received one item and not the other  as both were in the same box. 

I have the Post NL receipt.

Pls advice.