3rd party developers maing money on integrations over and over again

This is kind aproblem I dont know how to solve here. I know at least about 2 3rd party developers here who have done already some integrations on a custom work basis but dont list that fact in 3rd party integration in product checklist and rather selling that integration over again

Example:

customer: I need you product x to be compatible with product y

dev: no problem I will do it as custom work for 300 USD

Now the question if that work is already done is it ethical to kepp same price for another customer asking for exactly same integration if work was already done ?

I know this is very questionable theme ..but I think some ethic should be keep. If developer already make some demanded customization it will be fair to mention this on product page and for example set some fixed price for it ( I mean lower price than full price for custom work as integration is already done)

Same its wit prouct where there have geeb proven incompatibility with other 3rd party product. EVene if developer know that they not list that facts anywhere that its already proven that product x dont work with product y

I am of the opinion that if you charge for custom work, that work becomes the rightful property of the person that paid for it. If you are going to sell it to multiple people or if you plan on selling it to multiple parties, then the price should be the same for all and that fact should be released.

Yes I think same Sky... but most of developers dont do that and  and if another customer ask for same integration they already did they charge again high price as if they work on it from scratch.I understand its easy money for dev but from ethical point its not good work this way.. I think for most demanded integrations which are asked many times by customers on a developer , dev should release this iontegration for fixed price for all ...

You could be right, but if i ask for custom work, i don't want others to have that as well. it's custom work i pay for it i own it. :) 
Wouldn't make much sense if i paid $200 for something and the next guy get's it  for $50

I know but if somebody made some special custom work u are correct. But Im talking about most demanded integrations which for example 20 people or more ask for ...and developer charge price for custom work for all of them separatel as he do it from scratch but in fact all work is done already...

woops didn't see it was about integrations  lol sorry :)