The current episode group system does not support the same episodes in different sub-groups. If you already added it to a different grouping, there's no much you can do about it. Is there any particular reason why this group you are editing requires the same episode twice?

I see. I was just following the Wikipedia information (Japanese) about this re-broadcasted program of Gintama's main episodes.

The fourth season seemed to be using episodes that were already in past 4 seasons.

Do you think I should keep the fourth season without several episodes (which will technically mean wrong information)? Or probably create a new episode group for only the fourth season?

Quite honestly I don't know. You coud write in the description a small note indicating where the repeated episodes should be placed. There's nothing wrong with creating a separate group for the fourth season either. Both seem good options to me.

I guess I will choose to delete the fourth season (5th season in TMDb). Could you do that?

Done.

Sorry for my confusion. I wasn't clear enough.

I meant the next season of the season you deleted. Like I said, could you please delete the former 5th season (current 4th season) on TMDb? The season I want you to delete has currently 9 episodes.

By the way, I have a question. What language should the episode group title be? I couldn't find such guidance in the bible.

I will add the season you have deleted if I have a chance.

By the way, I have a question. What language should the episode group title be? I couldn't find such guidance in the bible.

Yeah, there's no much about it in the Bible yet, it's pretty much still a baby feature. I don't see any problem if you want to make the grouping in Japanese .

The fourth season of this has 4 more episodes to add, however, I couldn't add them probably because they are already added to the past seasons. What should I do??

FYI episodes can be added to multiple groups when needed. Episodes automatically disappear the list on the right when they're added to an episode group. So to bypass the system, you would first need to remove the episode from the episode group. Then, open the two seasons in different tabs and you'll be able to add the same episode to both.

What language should the episode group title be? I couldn't find such guidance in the bible.

English, like most field that do not yet have a translation: https://www.themoviedb.org/bible/general#5af1bddbc3a368762c000001