I had the exact same feeling. I saw the advertisement for this game only to discover that I already own this somehow. It looks like Eets Munchies was featured in the Humble Indie Bundle 9 back in September 2013. I tend to insta-buy any of the main Humble Bundles, so it doesn't surprise me that I wouldn't know I have some games.

I'm getting the same feeling lately. Simplest way to figure it out:
Go top/right in Steam client and click your account -> Account Details -> STORE TRANSACTIONS -> SEE ALL; then press CTRL+F and input game name or part of it.
And BAM! I got it almost exactly 6 months ago in a HB, seeing there are another four games with same date of purchase :)
And if you are buying from multiple bundles and don't know which one you bought it from, well I don't know how to track those easily. Maybe it's best to check your email within the same date range.

Use this site (http://igbwiki.com/wiki/Eets:_Munchies) for tracking bundle games and where you got them from.

It came from a game bundle. Don't remember which one, but yeah.

Same here.
The point is that back in the day, in the humble bundle, it was Eets Munchies Beta, not a complete version, so it went unnoticed.

I guess you got it in HB 9 along with Bastion, Brütal Legend, FEZ, FTL, Limbo, Mark of the Ninja and some other games

My quality of life has severely declined since noticing this game in my inventory