The amount in Box 20 (RPP contributions) of your T4 slip will only be shown in Box 207 of your income tax return and not in Box 208.  Box 208 will show amounts contributed to an RRSP or a PRPP.  You may have inadvertently entered the same amount shown in Box 20 again as an RRSP contribution in the RRSP section and as a result, ended up making a double claim for the amount shown in Box 20.