title: Xcode 11 - Instance method ... conflicts with same method from another category
body: <p>after updating to Xcode 11 all of my projects report many warnings (sometimes 1000+) like:</p>
  
  <pre><code>Instance method 'a_custom_method' in category from 
  /Users/XYZ/Library/Developer/Xcode/DerivedData/PROJECTFOLDER/Build/Intermediates.noindex/PROJECT.build/Debug-iphonesimulator/PROJECT.build/Objects-normal/x86_64/ProductCell.o 
  
  conflicts with same method from another category
  </code></pre>
  
  <p>As far as I can see these are custom helper instance methods I've created, but never had any issue like that before.</p>
  
  <p>As a result of these numerous warnings, I'm having difficulty compiling and running the project.</p>
  
  <p>I've checked similar older questions for this (eg. saying there may be #import ".m" file somewhere) but this is not the case for this.</p>
  
  <p>Anyone with similar experience? 
  Thank you!</p>