My wife and I have this issue as well we have 2 X5s and we each have our own key for both. We have 4 different cases, she has red and a white and I have a black and my ///M case. We also put stickers on before we figured the cases out, I had stickers with a man on them so those are on my fobs.
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X5M, Eisenmann exhaust, 22" HRE wheels, catless down pipes, Stage II tune/X5 Diesel, Lowered & Tuned