In some languages, the distinction between singular and plural of a noun isn't obligatorily marked at all. There are ways to be explicit about singular/plural but they are optional. (English adjectives use the same forms in singular and plural, too, while another language might mark the grammatical number at both nouns and adjectives. For yet another English-based example, patent law tends to treat singular and plural as interchangeable in some contexts.)
If a language strictly distinguishes singular and plural nouns, the basic, "unmarked" form is usually the singular one, and the forms for other grammatical numbers (plural, dual, trial, paucal...) are derived from the singular one.
Usually, but not always. Some Arabic words have a so called "collective plural" as their basic form, and the singular is derived from it. Interestingly, you can then sometimes take the singular form and derive from it a "count plural" - a form different from the collective plural, and often with a difference in meaning; the details depend on the dialect (I mean, on the language - spoken Arabic is a family of rather different languages) and on the word in question.
Example: onions - بصل [basal], onion - بصلة [basila]
You can find more and better elaborated examples in this thesis, Section 5.12.1.
The Arabic جن, "jinn" is such a collective noun, too.
Consider the English word form "fish" which has a variety of singular and plural meanings, and "fishes" which is used for certain other plural meanings - such as when referring to multiple species of fish, with a focus on studying the various species.
Some rather different parallels from English:
- choir -> choirist -> "choirists"
- (the) English -> Englishman -> Englishmen
Of course, interpreting either of these as a singular form derived from a plural form of the same lexical item would be highly non-standard; this is a different mechanism. But perhaps it shows how the collective plural can differ from a count plural semantically in a language where the two are different forms of the same word.
However, these two are not the only mechanisms through which a language may end up with a word whose singular form looks like its plural form with a suffix. The same may also be a result of diachronic evolution where a plural ending evolved into a null ending while the singular ending remained pronounced. Such a plural is perceived as a completely ordinary plural - the plural form is just shorter than the singular one. Native speakers might not even notice this as interesting or paradoxical, because if this occurs in a language where every noun form has a grammatical ending, there can be on average no correlation between the length of the ending and the grammatical number of the noun - there's no iconicity of plural marking in the paradigm.