Is this a security flaw for Other, Others, Others... accounts having 0 (root) UID? When running this command dscl . -list /Users UniqueID, I get duplicate UID, for the Other, Others, Others... accounts, which is the same UID root uses.

daemon              1
Guest               201
myusername          501
nobody              -2
Other               0
Other...            0
Others              0
postgres            502
root                0


I'm worried if this might be a security issue after I've updated from Yosemite → El Capitan (10.11.1), and I'm not aware if it was present before the update.
I've found out about this after being unable to install Parallels Deskop 10. The message from parallels was:

ERROR: 15476. Cannot install Parallels Desktop because there is a non-root user account with the UID 0 in your system

So this leaves me questioned whether it's a Parallels issue or the permission configurations.
Each user on OS X should have a unique identifier.

Given that your Mac is not managed and that you do not want the three Other accounts, reassign these accounts to different UID values – or delete them.

See Accidentally created user with duplicate UID. How to fix? for steps on how to reassign or remove these errant accounts.

This dscl command should work and form the basis for removing the other two accounts:

dscl . -delete /Users/Other


Be careful and please take a back-up first!