Currently, when you homebrew an item that requires attunement, you are invited to write the attunement requirements in the Attunement Description box. The hovering text box (when you hover with your mouse over the "?") suggests the following example: "Attunement Description - by a druid, sorcerer, warlock, or wizard."

If you type out -- as suggested -- "by a x, y, or z" in the box, the finalized version will show: "Requires attunement by a by a x, y, or z"  (double "by a").

This error appears often in the homebrew items section of D&D Beyond, because some homebrew content makers don't realize they need to go back to editing and remove the superfluous "by a" that they wrote following the given example.

My suggestion: instead of  "ATTUNEMENT DESCRIPTION", the bold title could read "REQUIRES ATTUNEMENT BY A..." followed by the blank field to type in the requirements (if any). 

(Or change the hovering text example to "Attunement Description - druid, sorcerer, warlock, or wizard ('by a" will be added automatically)"

Small quality of life change. 

A quick look at the first two pages of published homebrew magic items shows the issue is real... 

"Weapon (longsword), artifact (requires attunement by a A creature who chooses to attune to "Judy" ..."

"Wondrous Item, artifact (requires attunement by a Requires an artificer with high knowledge of religion)"

"Weapon (shortsword), artifact (requires attunement by a Requires Attunement By A Rouge)"

"Staff, uncommon (requires attunement by a Requires attunement by a Sorceror, Warlock, or Wizard)"

It would help if people thoroughly playtested and edited their homebrews prior to publishing too.

Quote from IamSposta >>

It would help if people thoroughly playtested and edited their homebrews prior to publishing too.

 Indeed :)