Mmh, hard pass on refilling butane gas canisters.
In order to get the gaseous fuel to migrate from one bottle to another, you have to heat one of them up to create a temperature differential.
Those valves and seals are designed to work one way, to release gas out of the canister, not let it migrate back in.
If you are concerned about getting value for money out of camping fuel, run an MSR or similar pump bottle cooker on kerosene (white gas).
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