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I saw that duelbits send reward from time to time to gamblers who are active on the platform. All you need is to check notification centre more frequently. Know that rewards have expiration time. But never heard that bonus have wagering requirement. However, this looks fair, but to get 5 bucks gambler must wager 5k - this looks unfair

I don't think that it is unfair. It is free, afterall. I just hope that they make it lower. A jumped from $1000 to $5000 is so hiighh. Maybe around $3000, or maybe $2500. Anyways, it's on descrition. And it's free, so shouldn't complain.
This is their promo codes from Telegeram, you can check them out.
Absolutely...it is quite normal to have wagering requirement in order to claim bonus drop. $5 with $5k wagering requirement is still considered as reasonable because there can be higher wagering requirement in other casinos for the same bonus drop. It looks increased much from 1k to 5k but I believe there is a reason for this adjustment. Nothing is completely free, bonus drop is a bonus for our gambling activity only and we should not force ourselves to wager just to be eligible for it.