licking spoon and putting back in the food
Safety-wise: If the food is heated at least to 180° F/ 80°C, I'd like to cite Jefromi


  But if you're just cooking for yourself I personally wouldn't really
  worry about it, because in general you'll be cooking the food at a
  safe temperature, not just above the danger zone (140F/60C) but
  something safe for all meat (180F/80C) so whatever bacteria you put in
  with your spoon is just going to get killed. We routinely put
  potentially contaminated things like raw meat into our food while
  cooking, and let the heat take care of it; whatever you might be
  carrying isn't any more dangerous.


and logophobe


  Double-dipping isn't a big concern unless
  you're ill [...]
  You're probably introducing your friends and family to just as much
  contamination in the form of dust and such simply by having them in
  your home.


Quality-wise, as Stephie already pointed out: Liquids thickened with starch will liquidify if these come in contact with saliva (the enzyme amylase in particular). It breaks down starch to sugar.