Also to add to that, if you only work for one employer, they should know that your MAX Social Security tax(1 of the 2 components of FICA) that should be withheld for any person is 6.2% of your first $117K; so if any individual employer withholds more than that, they owe you money back, not the IRS. IRS refunds stem from any excess withholding other than that.
Obviously if you have two jobs, the employers have no idea what's being withheld from your other job, so in that case your TOTAL SStax withheld may be more than 6.2% of $117K and the IRS will refund you. Still, if one of em alone withholds more than the MAX<–apply same rule as above.
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Sorta like labor laws: max hours a boss can you work you are 60hrs/week; but that doesn't stop you from getting 2 60hr jobs. Not the employers' duty to make sure you work a total below 60, but it is their duty to make sure they don't specifically work you more than 60.
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Your W-2(form that you+IRS get from employer after January and before 4/15) summarizes your total wages, state taxes withheld, federal taxes withheld, FICA taxes withheld($$ for SS tax, $$ for Medicare). Let's ya know all the math, so you can just compare numbers to make sure no errors. Definitely take a look at one of these. It's usually the first step in preparing your own return, unless you don't have a “boss”. <–Contractors, business owners, etc. (nobody to do those W-2s)
Employer provides W2s to you because it allows you to start your filing. They provide it to IRS so bigbrother can verify not only your numbers, but also the employers'. Two birds, one stone.