Double-taxation is effectively the penalty that you pay for the excess employee contribution whether the excess contribution is made to the traditional account or to the Roth account.
Treasury Decision 9324, Internal Revenue Service, 2007-22 I.R.B. 1302 states:
Designated Roth Contributions as Excess Deferrals
Even though designated Roth contributions are not excluded from income when contributed, they are treated as elective deferrals for purposes of section 402(g). Thus, to the extent total elective deferrals for the year exceed the section 402(g) limit for the year, the excess amount can be distributed by April 15th of the year following the year of the excess without adverse tax consequences. However, if such excess deferrals are not distributed by April 15th of the year following the year of the excess, these final regulations, like the proposed regulations, provide that any distribution attributable to an excess deferral that is a designated Roth contribution is includible in gross income (with no exclusion from income for amounts attributable to basis under section 72) and is not eligible for rollover. These regulations provide that if there are any excess deferrals that are designated Roth contributions that are not corrected prior to April 15th of the year following the excess, the first amounts distributed from the designated Roth account are treated as distributions of excess deferrals and earnings until the full amount of those excess deferrals (and attributable earnings) are distributed.
