In Russian we have: Why did you take a lot of clothes and equipment? Are going to go to war? But what about English idiom?
The classic idiom for taking a lot of stuff is "everything but the kitchen sink"1. There is an implication that a lot of the stuff will not be needed, i.e. the idiom is a bit derogatory.
If you want to say that someone is using a lot of stuff but without the implication of packing/travel, there's also "everything from soup to nuts".
1 When we go camping for two weeks each August for a medieval recreation event, this becomes "everything and the kitchen sink". :)
Idioms for bringing lots of stuff:
Particularly with subtle verbal cues and body language, these phrases could all imply:
Meaning: everything or completely.
Idioms for preparing thoroughly:
A baseball metaphor meaning prepare for every situation.
Meaning: Pay attention to the smallest details in doing a job.
In the classic Around the World in 80 Days, Phileas Fogg was known to say:
He always seemed to have just the right tool packed to solve any unexpected problem.
Truncates Hope for the best prepare for the worst.
Sarcasm works well here.
Is that all you're taking?
There really ought to be a simple idiom along the lines of "packing for a six-week picnic" or "bundling up in April for next winter" but I'm not aware of any existing English idiom that expresses precisely the right sense of excessive or premature preparation. Marthaª's suggestion of "bringing everything but the kitchen sink" comes closest, I think.
Wolfgang Mieder, A Dictionary of American Proverbs (1992) cites three not-very-well-known sayings that are somewhat relevant. Collected in Ontario:
Overdone is worse than underdone.
Collected in New York (and noted in Chaucer's "The Yeoman's Tale"):
Too much of a good thing is worse than none at all.
And collected in Mississippi:
There's nothing like too much preparation to dull the sharp edge of a man's honin'.
Along these lines, Charles Spurgeon, The Salt-Cellars (1889) reports a couple of sayings that offer variations on the idea of "too much of a good thing:
Too much oil puts out the lamp; too much wood puts out the fire.
and (my favorite):
Too much pudding will choke a dog.