Filing multiple H-4 and EAD petitions

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Hi,

I am currently with employer A since 2013. I am planning to move to employer B who is filing my H-1B, my spouse's H-4 and H-4 EAD (in premium) this week (hopefully). I also have an employer C who is also filing my H-1B, my spouse's H-4 and H-4 EAD (in premium) next week. Is it necessary to file H-4 and H-4 EAD along with every H-1B petition? Or would the H-4 (and EAD) petition(s) going through employer B suffice? And my other question is how do I tell employer C that I already have an existing H-4 petition?

And also, there may be a chance the employer A may also file the petitions this week or next (I am trying to get them to hold off) 

Thanks in advance

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Also, on the same line...is it ok to file H-1B petitions through multiple employers? Or is that an issue now a days?