The martian atmospheric D/H value of 5.2 times terrestrial is significantly higher than any found on Earth, and has been ascribed to preferential loss of H relative to D from the atmosphere through Jeans escape over time. Here, based on ion microprobe analyses of apatite grains from martian meteorite QUE94201, it is shown that the pre-Jeans escape martian water reservoir has a D/H value ~twice that of terrestrial water, rather than the terrestrial value that has been assumed in prior work. The data support a two-stage history for martian volatiles in which early hydrodynamic escape enriched martian water to ~2x terrestrial D/H values. Subsequent Jeans escape to produce the current atmospheric values has thus been responsible for less D-enrichment than previously thought. A martian crust containing 2--3 times more water than previously proposed is implied by the results.