It depends on your systems configuration. By default, most systems I know will configure
useradd to create a usergroup. See
man 8 adduser:
ADDUSER(8) System Manager's Manual ADDUSER(8)
adduser, addgroup - add a user or group to the system
Add a normal user
By default, each user in Debian GNU/Linux is given a corre-
sponding group with the same name. ...
Same holds for the low-level
USERADD(8) System Management Commands USERADD(8)
useradd - create a new user or update default new user
useradd is a low level utility for adding users. On Debian,
administrators should usually use adduser(8) instead.
By default, a group will also be created for the new user (see
-g, -N, -U, and USERGROUPS_ENAB).
As long as you only use
deluser, you shouldn't come into a situation where the group already exists.