I would say we use "dubious" most when referring to information, or sources of information. And suspicious when referring to people, or circumstances/coincidences. I think if you were to describe someone as "dubious" meaning "suspicious" it would definitely sound more formal. And even then, I would say something like "He looks dubious," or "He's dubious looking." Not just "He's dubious" whereas you could say "He's suspicious," and it sounds fine. Another use of "dubious" that is used that appears in the definition given by Google is "morally suspect." I think you just need to remember the contexts in which you see both words used and copy the natives. But they aren't interchangeable in 100% of cases so ten cuidado!

I can’t think of a sentence where I would reasonably substitute ‘dubious’ and ‘suspicious’. But they’re complicated, and used incorrectly by native speakers, too. For example He’s acting suspiciously. (In a way to make me suspicious of him) He’s acting suspicious. (In speech, this would probably mean I’m suspicious of him (and be criticized) but could mean that he’s suspicious of someone else) To me ‘dubious’ pairs more closely with ‘suspect’, not ‘suspicious’. For example His explanation is dubious. His explanation is suspect. The following are similar, too: I doubt his motives. I’m suspicious of his motives. His motives are suspect.

There is no doubt the two words are close in meaning in many circumstances, so there is some overlap. I have long tutored for college and other entrance exams, on which there is often a premium placed on subtle differences in meaning, and perhaps this might help: If a person looks dubious, the primary meaning is doubtful (you can see the same root in the words), for example, "I told him the watch I was selling was made of solid gold but he gave me a dubious look." Meaning, he had some doubts that what I was saying was true. A secondary meaning, often when applied to a thing or a situation, means of doubtful quality or questionable, for example, "He made the dubious assertion that the watch was made of solid gold." Suspicious is less about a sense of doubt and more about a sense of the (potentially evil) unknown. For example, "My neighbor was extremely paranoid and gave everyone who walked past his house a suspicious look." Meaning, he suspects some evil or untrustworthy motives. When applied to a thing, it has a similar sense, as in "Whenever the police find a suspicious package on the subway they must evacuate the entire train until they can verify that there is no danger." Reasonable people may differ, and there are slangy ways to use either word, but I think the core issue comes down to doubt (dubious) versus the general threat of the undesirable (suspicious). Hope it helps!

Often we do have two words that are very similar because one might come from French or Latin while the second one might come from Germanic or a nordic language. At one time England was like two countries with different languages, but eventually merged, so there are duplicates that have small differences ... About your question this might help https://wikidiff.com/dubious/suspicious

Agreed!! So much of this is subjective. I think dubious is a bit more literary and a bit more sinister. A dubious plot would make me think of lots of planning and evil machinations. But that may just be me!