To be clear: The employee is supposed to inform his employers. This does not mean the employer will do so.
I am curious too: How easy is it for an employee working for two or more companies to make excess deferrals; obtain the company matches; and get on with his life worry-free?
My take: If the employee withdraws the excess deferral after the April 15 deadline, then yes, he's double taxed. Plus AFAIC the plan administrator will withdraw the two employers' matches. All is well and good for anyone interested in fairness.
If he does not withdraw (meaning he does not inform either employer of the excess deferrals), then his W-2s may flag the IRS that he has excess deferrals.
This pre-pandemic 2018 article says the IRS will "conduct targeted audits for taxpayers who appear to have excess
401(k) deferrals, especially those with multiple 401(k)s":The only entities monitoring that an employee is inside the limit is the employee him/herself and in theory, the IRS, via W-2 reporting.
If the IRS sees the excess deferrals; audits the employee; and determines there was willful tax evasion, then the employee could face criminal prosecution. To review (using verbiage from the net):
"Section 7201 of the tax code creates the federal crime of tax evasion. The crime of tax evasion has historically served as the principal tax revenue offense.
There are two potential offenses under section 7201: (A) the willful attempt to evade or defeat the assessment of a tax, and (B) the willful attempt to evade or defeat the payment of a tax."
I suggest advising Jones that the sum of his W-2s' Box 12, Code D amounts, will exceed the 401(k) limit for the year. If the IRS is paying any attention, it will notice this. Then employee Jones is risking an audit. For said audit, it seems to me Jones ought to hire an attorney.
As for an attorney's role here, I think the client should be told that, at this point, failing to withdraw the excess deferrals at his own initiative could be seen as a willful attempt to evade or defeat the assessment (or payment) of a tax, and he could be criminally prosecuted. Penalties include jail time of up to five years and up to $250,000 of fines.