On textarea.org, copy-paste many paragraphs of text from somewhere, place the cursor at the end and hold down backspace.  Occasionally, a word is duplicated when the cursor reached the end of it.  Video attached.  (Note that sometimes it can take about 20 seconds of holding backspace before a repro.)

Video was taken on Chrome 53.0.2746.0, MPE24.49-18, Moto X 2nd gen, IME thread enabled in chrome://flags, Google Keyboard 5.0.25.122319759-arm64-v7a.  Reproduces regardless of whether IME thread is enabled or disabled.
 

Deleted: screenrecord-20160601_123720.mp4 832 KB

screenrecord-20160601_123720.mp4 832 KB Download