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Hi Jens-Hermann. You will note I said "probably". ;)
C.palustris usually have pale halteres, C.curvicauda are usually dark.
C.palustris have green or bronze reflections on the abdomen, C.curvicauda have pink or copper reflections.
Of course I like to check genitalia to be sure. Female ventral receptacles are useful - curvicauda has a narrow "neck" before the cap, palustris lacks the "neck".