Some emails don't arrive or duplicate in 2 folders

I want to find the best email client - outlook, thunderbird or eM Client.
I’m using now gmail and as client outlook (not satisfied). Yesterday I installed Thunderbird and EC.
On my ideapad 330S, 8 GB RAM, 3 GB empty space on C, but email Data on D 300 GB empty.
When I installed EC yesterday, using outlook for data collection of 5 gmail accounts and 1 outlook account, which was almost okay, on some archive folders arrived in the wrong account, which I repaired manually.
Whether all emails had been stored in the respective folders I didn’t check yet - too many.
After one day testing it looks like EC pro is the best of all, but please tell me first how it will function without the following issues:
There are 5 folders for gmail acounts and 1 folder for outlook, let’s name them A to F…
A shows 6 emails arrived, which is correct, but in B it shows the same 6 like in A. Newly incoming emails to A arrive also in B. But in B,C, D, E and F must be 1,0,15,10, 1 as in gmail.
To click the sync button will not help.
How can I solve this problem?
Many thanks to all of you who will let me know the solution, or even who try to find it.

Sounds very weird that’s happening. Normally if you add Gmail and Outlook IMAP accounts whatever label / folders you have setup online are mirrored under the same accounts in eM Client.

So if you are seeing different label / folder duplicate messages and also some emails don’t appear in eM Client that are online, then I would remove all your accounts and just start by adding one IMAP account at a time and see if that works.

Thanks cyberzork, this might work.
However I will wait whether there are some more suggestions.
Do you think the reason of the issue might be because I was installing with outlook data even I have problems with contacts and calender there?
But both, contacts and calender are okay in EC.

Should it be better to use gmail for importing?