The T5008 and T5 you were issued may relate to the same investments but both need to be entered as separate slips. The T5008 relates to dispositions of securities held in this investment, and the T5 relates to interest, dividends and other "investment income" earned on that investment.

On the T5008 slip, you need to enter a figure in Box 20. This would be the cost of the fund or Adjusted Cost Base (ACB) and if you do not enter it, you will be paying Capital Gains on that entire amount shown in Box 21.