Martin (1992: 301):
Setting aside the fact that for many texts these fine differences in meaning do not matter, it could be argued that the only true synonymy is repetition, including a lexical item's formal scatter (e.g. defeat, defeats, defeated, defeating). But even here, an item's derivational scatter gives one pause. Looked at from the point of view of repetition, the difference between synonymy and co-hyponymy can be thought of as an [sic] a kind of scale, graded along the following lines:
same meaning defeat:defeat repetition defeat:defeats inflectional difference defeating:defeat derivational difference defeat:down synonym defeat:trounce attitudinal difference defeat:subjugate co-hyponyms another meaning
 In SFL theory, synonymy is a means of lexical cohesion, where 'a speaker or writer creates cohesion in discourse through the choice of lexical items' (Halliday & Matthiessen 2004: 570).
 Synonymy and repetition are both relations of identity; that is what they have in common. In synonymy, the token and value are different lexical items, whereas in repetition, the token and value are the same lexical item; that is how they differ. Repetition is not 'the only true synonymy', it is a limiting case of synonymy.
 This confuses the two abstractions, lexical and grammatical, of the notion of 'word'. The variation in the form of words according to their grammatical function is the word as one the ranks in grammatical systems, not the word as lexical item or lexeme. See Halliday & Matthiessen (1999: 568).
 The difference between synonymy and co-hyponymy is that synonyms are similar in meaning, whereas co-hyponyms are members of the same class. Dogs and cats are co-members of many classes (domestic pets, carnivores), but they are not synonyms. Cf co-meronyms: parts of the same whole (brain, stomach).
 The 'grading' confuses a number of variables, including lexis vs grammar, and experiential vs interpersonal meaning.
 The difference between 'defeat' and 'defeating' (note the change in order) is inflectional rather than derivational, except in the comparatively rare case where the latter functions as a nominal.
 The words 'defeat' and 'trounce' are also synonyms (experiential meaning), whatever their difference in attitude (interpersonal meaning).
 The words 'defeat' and 'subjugate' are also synonyms; a specific class of which their meanings are both hyponyms is less obvious.