I was running into this error with TP-Link Smart Plugs. Here's how I got in the situation and how I fixed it.
I had a smart plug called "Guest Bedroom". I was able to turn on it on by saying either:
"Alexa, turn on Guest"
"Alexa, turn on Guest Bedroom"
Then I added a few more smart plugs and decided to change the name of that device from "Guest Bedroom" to "Guest Bedroom Light". There were no other devices named "Guest" or "Guest Bedroom" or any devices that started with those. There was only "Guest Bedroom Light" now. After making this change, saying:
"Alexa, turn on Guest"
"Alexa, turn on Guest Bedroom"
I got the dreaded "A few things share that name, which one did you want" and I had to specify the whole name "Guest Bedroom Light". Not a big deal, but frustrating, because I know there's an entry in a database somewhere that's no longer needed, but it's stopping me from using the abbreviated versions with Alexa for my guest bedroom light. I figured I needed to somehow reset that database.
Here's what finally worked.
1. Start Alexa app on iPhone (make sure you have the most current version of the App)
2. Tap Device icon on lower right of screen
3. Tap the "+" icon in the upper right to add a new device
4. Tap "Add Device" (Even though there is no new device to add)
5. Tap "Plug"
6. Tap "TP-Link Kasa"
7. Tap "DISCOVER DEVICES"
And that fixed it. My guess is in trying to discover devices Alexa/Amazon rebuilt the database and all the old names that no longer existed were discarded. Don't know if this will work for everything, but it worked for TP-Link Smart Plugs.