Recently had some guest at my house - while I was in another room one of the guest was sampling the food repeatedly and put the spoon from mouth back in the food. Is this a health concern? Does the food spoil faster?
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.
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.
There are many health concerns with sampling food with a spoon and putting it back in the food. Saliva contains microorganisms/bacteria that may be harmful to us especially high risk individuals(pregnant persons,the elderly,those who are Ill) Pathogens can cause food borne illnesses or infections. Therefore it is better to practise safe food handling methods.
Yes I would say there is a problem with sampling food and then putting the spoon back in the food. The food would now be contaminated with any bacteria this person was carrying around in their mouth. Not to mention the germ factor. As to spoiling faster I don't think that would make a difference. Maybe you should talk to your guest about doing this.