Sabz
Tell them it's bad for SEO and they'll stop immediately ;)(It's frowned upon to have different link which target the same page - true for WAI conformance too)
On a serious note - do a Krug hallway usability test with your family, friends and document some evidence as why it's a problem.
If you don't do this it will be your opinion (how ever well informed) against theirs. Also if possible offer them a suggestion about how they can achieve their goal without compromising the usability.
Requires a bit of thinking but will help to promote user experience thinking into the business.
All the best,
Matt