add error message for duplicated case names

[pettermahlen]

Fixes #11

[codecov-io]

Codecov Report

Merging #12 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master      #12      +/-   ##
============================================
+ Coverage     94.72%   94.78%   +0.05%     
- Complexity      178      179       +1     
============================================
  Files            21       21              
  Lines           550      556       +6     
  Branches         79       80       +1     
============================================
+ Hits            521      527       +6     
  Misses           22       22              
  Partials          7        7

Impacted Files	Coverage Δ	Complexity Δ
...potify/dataenum/processor/parser/ValuesParser.java	94.11% <100%> (+3.2%)	5 <0> (+1)	⬆️

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