I came up with the following rule that I hold myself to:
If you come across a thing that you forgot you had, give / throw it away1.
Even if your reaction to finding the object again was 'OMG, how could I forget about this!! I want to keep it' - throw it away.
The fact that you've forgotten about it is a clear sign that you wouldn't have missed the object, had you never found it again. So, don't let the rediscovery of an item prevent you from letting it go.
Also, 'I paid for this' or 'I got it as a present' aren't valid reasons to keep the items. Don't let this kind of thinking be the sole reason of you keeping some object you don't want.
Decluttering feels good.
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If the item could be useful to someone else, consider selling, gifting or donating it. ↩