As you can see in the picture below, the statistics for Flying Monkey are doubled when you add it to the extra menu in your character sheet.

https://imgur.com/UTbIqsk

I can tell by the symbols that you are using the 3rd-party extension, Beyond20. It is known to occasionally cause issues such as that. Disabling the extension should resolve the issue for you.

You can read about everything I tried below (do you need a image for everything?). The problem seems only to be connected to the Flying Monkey in the extras menu, which I bought from "Tomb of Annihilation", as a Individual Purchase Item.

Additionally, have you experienced this with any other extra, or just the Flying Monkey?
- Tried adding other monsters (all under "A" & "F") as a familiar, no problems. 
- Tried adding the Flying Monkey under all the other extras categories, the problem remained

Are you running any browser extensions, even ones that aren't meant to interact with this site?
- Turned of all extensions, the problem remained

Does the issue occur when you use a different browser, or on a different device?
My Computer (Windows)
- Google Chrome, the problem remained
- Microsoft Edge, the problem remained
My phone (Oneplus 3)
- Google Chrome, the problem remained
- Ecosia, the problem remained
- Firefox, the problem remained

If you have the App, does the issue occur on the app?
- Characters/Extras is not available via the app
- Listnings/Monsters/Flying monkey, no problems.

Have you tried adding the Flying Monkey to the extras of a different character sheet?
- Tried adding the Flying Monkey to 5 other characters of my own, the problem remained

Also Tried adding the following
Adding the Flying monkey as a extras to another users character sheet
- Viewed from my computer, the problem remained
- Viewed from their Ipad, through Safari, the problem remained

-Adding the Flying monkey to the combat tracker, no problem.

Confirmed:

I went to extras and tried to add flying monkey but it didnt come up on my D&D Beyond account though

Quote from Vladamir_Yuri >>

I went to extras and tried to add flying monkey but it didnt come up on my D&D Beyond account though

 

 
That monster is only available if you have access to the Tomb of Annihilation.