A doublet is one of two or more words in a language that share a common root word, but may have traveled into a language through different routes. For that reason, doublets may be nearly synonymous, but are not necessarily interchangeable. For example English pyre and fire are doublets. Subtle differences in the resulting modern words contribute to the richness of the English language, as indicated by the doublets frail and fragile (which share the Latin root, fragilis): one might refer to a fragile tea cup and a frail old woman, but a frail tea cup and fragile old woman are subtly different and possibly confusing descriptions. The English language is a West Germanic language that originates in England. ...
Another example of nearly synonymous doublets is aperture and overture (the commonality behind the meanings is "opening"), but doublets may develop divergent meanings, such as the opposite words, host and guest, which occur as a doublet in Old French hospes, before having been borrowed into English. Doublets also vary with respect to how far their forms have diverged. For example, the resemblance between levy and levee is obvious, whereas the connection between sovereign and soprano is harder to guess from the forms of the words alone.
Some doublets occur when a word is borrowed from the same language twice, such as warranty and guarantee, taking different routes from Old North French and Middle English.
A linguistic triplet is an etymological extrapolation with three results rather than the doublet's two. Consider a welcome from the heart, in English: a hearty welcome, a cordial welcome, or a sincere welcome. The Indo-European ker or the proto-Indo-European kerd (see http://www.etymonline.com/abbr.php for an explanation of proto-Indo-European) is the common root, from which the Latin cor, modern Italian cuore, French coeur -- and German Herz came into English. The differences may even seem to reflect an "ethnic variation" of a welcome "from the heart"--all are positive, yet, similar to the shades of meaning in frail and fragile, there is an appropriate place for a hearty welcome, a cordial welcome, and a sincere welcome.
Many doublets and triplets can be discovered by tracing etymology to roots of the proto-language family. The forward linguistic path also reflects cultural and historic transactions.