What does "My duh on X" mean?
Does it mean "I like X" or "I don't like X" or something else?
What does "duh" mean in general?
In this context, I believe it means something like "My mistake; I should have known that."
As @DavidM points out, "Duh" tends to be used to point out the ignorance or obliviousness of a statement as something that should be obvious:
"Wow, that engine gets hot after running for an hour."
"Well, duh. Of course it does!"
Saying "My duh" would imply that you are acknowledging that something should have been obvious to you.
"My Duh" means "my stupidity" or "my mistake", from what I can tell on my internet search. "Duh" is the sound we make to sound dumb - see link below for video examples. I imagine that it is a mashup of "my bad" and "duh".
For example: Duh moments
"Duh" is a word that represents a sub-vocalization. It usually is used on its own to suggest that the other person has said something stupid.
"My bad" is an idiom which means roughly: "That was bad, I did it, and I take responsibility."
"My duh on that" probably means the speaker is saying: "That was stupid, I did it, and I recognize that."
I've never encountered the term "My duh on ..."
Duh, in general, is a sound that is made to imply derision or feigned ignorance about the perceived stupidity or obviousness of a statement.
In other words, to say "You really don't understand something as simple as that?" It's mild, but impolite.
Example of usage:
"We really should have bought our tickets to the movie before it sold out!"
"Duh!?! You think?!?"