My Roth IRa is through my employer and the amount is entered on my W2. Do I enter this amount again on the "Enter Your Roth IRA Contributions" page?

If your Roth IRA contribution is entered on the W-2 in box 12, then you do not enter that contribution anywhere else on the tax return.

That's complicated.  What is the code in box 12 of your W-2?

The usual answer for this is that employer qualified plans are NOT IRAs, even though they have a similar purpose.  If you have a Roth account in a 403b, or a Roth account in a 401k, that is not an IRA, and it is only reported on your W-2 (code AA or BB). 

However, there is something new called a Payroll Deduction IRA.  This is where your employer contributes directly to your IRA, instead of setting up a qualified plan (which can have a lot of paperwork).

https://www.irs.gov/retirement-plans/plan-sponsor/payroll-deduction-ira

This is the same as a private IRA, except some of the money comes from your employer.  The contributions follow the IRA rules and limits (not the 401k rules) and are limited by income and filing status.   The contributions will not officially be reported on the W-2 (they could be in box 14, which is for notes and comments, but are not in box 12).  You need to report them as contributions. 

So it depends on whether you have your own IRA that your employer pays in to via the PDI, or whether you have a qualified plan that you called by the wrong name. 

What you have through your employer is almost certainly not a Roth IRA.  Amounts reported with code AA, BB, or EE in box 12 of your W-2 s a designated Roth account in a qualified employer plan (401(k) , 403(b), 457(b) or the federal TSP).  Nothing about amounts reported with these codes is to be entered under Deduction & Credits.

(Opus 17, Payroll Deduction Roth IRAs, while uncommon, have been around as long as there have been Roth IRAs.)