According to Longman Dictionary one of the meaning of the scent(noun) is:
[countable] a pleasant smell that something has
Synonym: fragrance:a yellow rose with a lovely scent
the sweet scent of ripe fruit
I'm wondering if the word scent already means pleasant smell, why "lovely scent" and "sweet scent" are used in the above examples? For example for the second one, according to dictionary, the word "sweet" also means having a pleasant smell.
Longman's definition isn't a universal one.
- A distinctive smell.
1: effluvia from a substance that affect the sense of smell
(American) 1. a distinctive odor, especially when agreeable
(British) 1. a distinctive smell, esp a pleasant one
3.a. An odour or aroma; a distinctive or characteristic smell; esp. one that is pleasant.
The sense of "pleasantness" is not a defining characteristic of "scent".
For example, the OED gives these examples (along with others) of the use of "scent" in real-world usage:
... the smell of rotting leaves, harsh acid scent of blood.
His throat constricted with nausea at the overpowering scent.
There is scarcely a scent odious or agreeable that may not be met with in the insect world.
Indeed, the first attested usage is of an unpleasant one:
?1473 Every man rose fro the table abhorryng & eschewyng the sente and sauour of the dede man.
In modern spelling:
Every man rose from the table abhorring and eschewing the scent and savour of the dead man.
It is true that "scent" is most often used in the pleasant sense. (Hence the usage notes by Dictionary.com and the OED.) So Longman is not strictly speaking incorrect. They accurately capture the meaning of the word that English language learners (their target audience) will most likely encounter. However, they would be incorrect to imply that "scent" is only used in the pleasant sense, and never for unpleasant smells. As the usage examples in the OED indicate, that's not the case.
Not every pleasant smell is "sweet". You could have a "spicy scent" or a "musky scent", for example. Moreover not every "scent" is pleasant. You can talk about "the foetid scent of the stinkhorn fungus". The word scent is often used for pleasant smells, but can be used for unpleasant ones too.
The word "lovely" serves to emphasise. All roses have a scent, but this rose has a particularly nice scent. Saying that it has a "lovely scent" gives information about this rose.
Sometimes words are used redundantly, but that isn't a mistake. It can help to clarify or emphasise.
Even the Longman dictionary has other definitions including "[countable] the smell of a particular animal or person that some other animals, for example dogs, can follow" Even accepting Longman as correct (and I have questions there...), additional qualifiers can be needed to make it clear you mean the scent is pleasant rather than just being capable of being tracked by animals.
Also, as James K correctly noted, humans often include redundant information when speaking, either for emphasis or clarity. Even if "scent" unambiguously implied pleasant, additional words could emphasize that it was very pleasant or in what way it was pleasant.
Like most words, scent has morphed in meaning over time. Etymology online gives the following history:
late 14c., senten, originally a hunting term, "to find the scent of, perceive by smell," from Old French sentir "to feel, smell, touch, taste; realize, perceive; make love to," from Latin sentire " to feel, perceive by the senses; give one's opinion or sentiments" (see sense (n.)).
So we still have the hunting term. We've frozen the meaning by naming the anal glands of a skunk "scent glands". And the term has been adopted by perfume makers to refer to their products, along with "fragrance". There's also "perfume" which can refer to the smell as well as the product or the act of applying the product:
The perfume of the Gardenias filled the room.
But I would not use that for a smell that was not flowery or at least something that might be used as a perfume.
Other words that might fit are "aroma" which is usually pleasant, but more often associated with food or drink. There is also "odour" (or "odor" in simplified English) which is usually unpleasant. And "reek" and "stink" which always refer to unpleasant odours.
