The issue is faced on XCode6 and above, Yosemite above.When one tries to use one developer account in two macs.
Lets get historical .
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In xcode 5 and below the standard way of making adhoc distribution build on two mac was.
1. download provisioning (assuming you know how to make certificate and provisioning).
2.Checkout certificate (as downloading will not work on other mac).
3. transfer them both to your second mac.
4. install them and run/archive and export.
If you have done so before with an account and now upgraded to Yosemite and using XCode6 you will face this error.
while exporting you will get prompt "Your Account already has a valid iOS Distribution Certificate"
Time to resolve this
The change is now you do not export certificate you export your entire developer account
.This is how you do it
1.Go to xcode->preferences ->accounts.
2.In accounts on the bottom left there will be a gear icon that will have option to export developer profile
3.Click on export developer profile and enter password of your choice.you will get a .Developerprofile
4. Now transfer this file using any thing to other mac.and after making archive when you export ipa for adhoc distribution wait for the prompt "Your Account already has a valid iOS Distribution Certificate"
click on Import Developer Profile button and select the .Developerprofile file we just made on other mac.
This is the new way of transferring provisioning and certificates :) sorry for the long post. here is a iPotato