Its definitely recommended to replace them in pairs and if one is leaking the other isn't far behind. At 200 for the pair it's probably the cheapest maintenance item you will come across in an E70.

Arnott makes quality replacement parts, my E53 has replacement Arnott fron air struts been flawless for over 2 years now.

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2018 Ram 2500 6.7L Cummins 68RFE
19k miles -Bright White/Black - Big Horn Sport - Crew Cab Short Bed
2013 X5 35D (CEO's) - Born on 5/17/2013 -
82k miles - Alpine White/Cinnamon Brown/Premium Pkg, Sport Activity/Premium Pkg and Sound/20" Style 214/Running Boards