The only type of SEP plan that allows employee contributions is a SARSEP plan established before 1997. If this is the case, your contributions are reported with code F in box 12 of your W-2. Is that what you have? If so, report this only on TurboTax's W-2 form, nowhere else in TurboTax, particularly not under Deductions & Credits.
Another possibility is that this is actually a SIMPLE IRA plan (not a SEP plan) with your contributions reported with code S in box 12 of your W-2. Similarly, report this only on TurboTax's W-2 form, nowhere else in TurboTax.
Some custodians permit regular personal IRA contributions to be made to a SEP-IRA account. Doing so through your employer is referred to as a Payroll Deduction Contribution and would be reportable by you on your tax return as you would any personal traditional IRA contribution that you make yourself. The payroll deduction is simply for your convenience, it is not an employer plan contribution. As such, it would be an excess contribution for 2016 because you reached age 70½ in 2016 or 2015.
