contradicting

[Ju]

Dave wants to join the party. At the same time, he doesn't want to join.
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1. Shall I say,

Dave has been very contradicting over making the decision if he is going to join the party of not.

2. What do we say this kind of charactor like Dave ?

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ju

[Route21]

Hi Ju

Personally, I wouldn't have used the term "contradicting". There are other words that would be more appropriate, such as "irresolute" or "indecisive".

Dave has been very irresolute over making the decision if he is going to join the party of not.

I would classify him as an indecisive character - a "ditherer" - always "blowing hot and cold"
See: dithering - definition of dithering by the Free Online Dictionary, Thesaurus and Encyclopedia.
dith·er (dr)n. A state of indecisive agitation.

intr.v. dith·ered, dith·er·ing, dith·ers To be nervously irresolute in acting or doing.

Hope this helps
Best regards
R21

[JMurray]

Dave has been very contradicting over making the decision if he is going to join the party of not.

Ju.
Dave has been in two minds about whether he is going to join the party or not.
Dave has been undecided about whether he is going to join the party or not.
Dave has been ambivalent/uncertain/unsure...

He is being indecisive.
He is wavering.
etc

not a teacher