Inside Alexa, if you click on the light, then on “edit”, you will see “Connected Via” and “Description” attributes. Sometimes the values that they display can point you to the mechanism that made them discoverable by Alexa.
For me, it was the Emulated Hue Bridge in Home Assistant - and as soon as I disabled it, the duplicates were no longer created… but I think the problem could also be caused by Homekit, the HomeBridge plug-in for HA or from another home automation hub (ie Wink or SmartThings). I also found that if I manually renamed a light inside of Alexa - the next discovery would rediscover it with the name I had assigned it in the Hue app (although I doubt that is your issue).
I’m not running Homebridge or Emulated Hue so I don’t think that is the issue. The duplicate is coming from Home Assistant and the only want to prevent it is to shut down my home assistant server. And it has also messed up my Alexa routines now. I removed the duplicates by shutting down the server but now alexa won’t do anything with Hue lights that are in routines. I’ve tried the forget all and discovery thing. I’m not sure what the deal is…any help is appreciated.
My solution (because I was running into the same problem with both Sengled lights and Hue lights) I would delete them only to have them show back up a few minutes later… was to simply add a number 1 beside the name… so for example you have 2 hue lights named Hallway… instead of trying to delete them change one of the names to Hallway 1 then when you say “ Alexa turn on Hallway” it will only recognize the one without the number