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Hi Juergen,

The answers to all your questions are: YES

Yes, it is normal to see many Dilophus in late summer,
Yes, they are D. febrilis (second generation)
and yes,
you see in some place more females.
Males form clouds as in Empididae, females are more frequent on flowers.
Yours are febrilis as well (females with dark wings, easy)

Theo Zeegers