With the help of others on scientificlinuxforum.org in resolving a software issue - 
gThumb was broken (see https://bugzilla.redhat.com/show_bug.cgi?id=1578517)- I tried 
to update from SL 7.4 to 7.5

That little escapade threw up another issue. A diagnostic was suggested:
yum clean all
yum --disablerepo epel check-update
yum --disablerepo epel update

(1)
# yum clean all
Loaded plugins: fastestmirror, langpacks
Cleaning repos: epel google-chrome nux-dextop repos sl sl-extras sl-fastbugs
: sl-security softwarecollections
Cleaning up everything
Maybe you want: rm -rf /var/cache/yum, to also free up space taken by orphaned data 
from disabled or removed repos
Cleaning up list of fastest mirrors
#


(2) **************
# yum --disablerepo epel check-update
ended with ...

Update notice SLBA-2018:1066-1 (from sl-fastbugs) is broken, or a bad duplicate, 
skipping.
You should report this problem to the owner of the sl-fastbugs repository.
If you are the owner, consider re-running the same command with --verbose to see the 
exact data that caused the conflict.
Update notice SLBA-2018:1424-1 (from sl-fastbugs) is broken, or a bad duplicate, 
skipping.
#


(3)
# yum --disablerepo epel update
ended with ...

--> Finished Dependency Resolution
Error: Package: pyexiv2-0.3.2-22.el7.x86_64 (@epel)
Requires: libexiv2.so.12()(64bit)
Removing: exiv2-libs-0.23-6.el7.x86_64 (@base/$releasever)
libexiv2.so.12()(64bit)
Updated By: exiv2-libs-0.26-3.el7.x86_64 (sl)
~libexiv2.so.26()(64bit)
You could try using --skip-broken to work around the problem
You could try running: rpm -Va --nofiles --nodigest

I got gThumb installed and running with a rebuilt pyexiv2-0.3.2-22.el7.x86_64.
Then I successfully ran yum update.

While everything has worked out for me I needed to draw your attention to the matter 
raised at (2).


Thanks

Jeff Shrowder