If you have RPP (Registered Pension Plan through your employer or union) contributions shown in Box 20 of your T4, you only enter them once in TurboTax, on the T4 screen. They will appear on Line 207 of the tax return as a deduction from your total income. If you imported your tax information from CRA using the AutoFill feature the entry will already be there.
NOTE: Do not confuse these contributions with RRSP (Registered Retirement Savings Plan) contributions. If you contribute to an RRSP through your employer with regular deductions from your pay, you will receive separate RRSP Contribution slips.