Stephen Hughes wrote:
Luke 18:3: χήρα δὲ ἦν ἐν τῇ πόλει ἐκείνῃ, καὶ ἤρχετο πρὸς αὐτόν, λέγουσα, Ἐκδίκησόν με ἀπὸ τοῦ ἀντιδίκου μου.
This is a good parallel to Acts 19:18. In both cases the imperative form indicates an imperfective aspect which in these contexts refers to an iterative action. In Luke 18:3 the same person did the same action repeatedly, and this is captured in most translations, e.g. NASB " she kept coming to him, saying" or NLT: "A widow of that city came to him repeatedly, saying".
In Acts 19:18 we have a plural subject so it seems likely that people arrived one after another to do the same action. Both the coming (ἤρχοντο) and the burning of the books (κατέκαιον) describe actions repeated by different people. NASB has tried to capture this by saying: "Many also of those who had believed
kept coming, confessing and disclosing their practices." However, I think the word "kept" works best if it is one person repeating the same action. When the subject is plural, it is difficult to express the iterativeness in English, so most translations leave it implicit to be understood from context. These people probably did not come as a unified group at the same time, but individually, one after the other. I suppose one might express the iterativeness by saying "they came one by one" or "one after the other".
Curiously, NASB says for the burning in the next verse: "And many of those who practiced magic brought their books together and
began burning them.." I think the iterative sense is more likely than an inceptive sense. (Inceptive is usually for the aorist.) Various people came one by one and added their books to the burning pile. The word begin suggests to me that they tried to retrieve the books when they were halfway burned, surely not the intended meaning.