Rachel Uchitel -- Overextended in Cover-Up

Rachel Uchitel -- the pilgrim of mistresses -- wore her finest gown to a 4-hour threesome in Beverly Hills yesterday ... dropping about $5,000 on a whole new 'do, including hair extensions.  But when you've got $10 million ....

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We're told Tiger Woods' #1 lady went to Norma Bouza -- of the famous Cristophe Salon -- to get locked up.

Locks are hair, right?