As a work around, I can unassign myself, then hit the Assign to All button.
Here's the error:
DBD::mysql::st execute failed: Duplicate entry 'keelerds-12pt1' for key 2 at /opt/webwork/webwork2/lib/WeBWorK/DB/Schema/NewSQL/Std.pm line 329.
The information below can help locate the source of the problem.
in WeBWorK::ContentGenerator::Instructor::assignSetToAllUsers called at line 58 of /opt/webwork/webwork2/lib/WeBWorK/ContentGenerator/Instructor/UsersAssignedToSet.pm
in WeBWorK::ContentGenerator::Instructor::UsersAssignedToSet::initialize called at line 192 of /opt/webwork/webwork2/lib/WeBWorK/ContentGenerator.pm
in WeBWorK::ContentGenerator::go called at line 353 of /opt/webwork/webwork2/lib/WeBWorK.pm