sounds like pokerus. is it shiny reshiram because then that would be fake. unless this is on a normal reshiram then its totally fine.
another Golduck random encounter shiny on a lake last time was in sinnoh
the only random encounter shiny was in one of the lakes and it was a golduck
it got traded later for another shiny. since then all my shinnies are are hatched or from trades
also i have many Pokemon up for trade just saying i want to get rid of the multiple Pokemon