Validate duplicate middleware load

[ad-m]

Protection for #986.

[kezabelle]

Looks sensible enough to me.

[matthiask]

To me as well, but the build fails with a flake8 error:
https://travis-ci.org/jazzband/django-debug-toolbar/jobs/362824956

Can you please fix this @ad-m and ping me after this?

[codecov]

Codecov Report

Merging #1052 into master will decrease coverage by 0.12%.
The diff coverage is 66.66%.

@@            Coverage Diff            @@
##           master   #1052      +/-   ##
=========================================
- Coverage   84.33%   84.2%   -0.13%     
=========================================
  Files          24      24              
  Lines        1296    1298       +2     
  Branches      176     177       +1     
=========================================
  Hits         1093    1093              
- Misses        155     156       +1     
- Partials       48      49       +1

Impacted Files	Coverage Δ
debug_toolbar/apps.py	89.74% <66.66%> (-4.86%)	⬇️

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[ad-m]

Fixed.

[matthiask]

Thank you!

[blueyed]

I think this should be a warning only.
See #1102.