iOS Development Signing identities in a tangle?

If you're using a developer account on only one Mac, you can get away with clicking "Fix issues" and letting XCode do hacky patch-ups to get things sorted.

Otherwise, you need to properly sort your certs out.

How to sort it out, the slash and burn way:

1. From now on, stop using the "Fix issue" buttons and inform everyone on the team likewise - a single "Fix issues" can break everything if it resets the certs.
2. Clear Keychain of old dev account keys on all Macs
1. Open Keychain Access
2. In top left sidebar, login
3. In bottom left sidebar, Keys
4. Find keys relating to dev account - these are based on the original certificate request naming, so it could be anything! There'll be a private key and an associated certificate, and possibly a public key. Likely a set of these for both Development and Distribution. Delete these.
3. Open https://developer.apple.com/account/ios/certificate/ and revoke all certificates
4. Generate certificate requests for both Development and Distribution on one Mac (this one will be the only one with public keys, from what I've found)
1. Open Keychain Access
2. Go to Keychain Access > Certificate Assistant > Request Certificate from a Certificate Authority
3. Fill in email address
4. Fill in Common Name with something like "{dev account email} - {distribution/development}"
5. Select Save to disk instead of "Emailed to the CA"
5. Create new certificates via https://developer.apple.com/account/ios/certificate/ for both Development & Distribution, following the fairly self-explanatory wizard. Download and double-click on the "master" Mac so that they're saved via Keychain Access.
6. Export the private key/certificate pairs
1. Open Keychain Access
2. In top left sidebar, select login
3. In bottom left sidebar, Keys
4. Find the keys, export with some one-time use password
7. Import the private key/certificate pairs onto the rest of the Macs
1. Get key files onto rest of Macs, double-click in Finder and save via Keychain Access
8. Create required provisioning profiles via https://developer.apple.com/account/ios
9. Set up dev accounts on all Macs
1. XCode > Preferences > Accounts
2. Add account, log in via credentials
3. Select the Agent user, view details
4. Download All - this not only does provisioning profiles, but seems to fix something up with the signing identities