It’s becomes clear from Bereshit 18:2 that Avraham is looking around from the entrance of the tent and sees three men appearing, cause it reads: והנה שלשה אנשים נצבים עליו. But what did Avraham sees the second time the word וירא appears? It only says ‘and he saw’, it doesn’t say ‘when he saw them’ (והוא ראה אותם) or ‘as he saw them’ (כפי שהוא ראה אותם). And although it becomes clear he ran to great/call/meet them, it doesn’t seems to be connected to the fact that ‘he saw’, for he already saw them in the first part of the passuk. So what did he saw the second time he looked that made him ran towards them instead of ranning towards them right ahead.
What does the repetition of this word וירא imply? The first time it has its ordinary meaning (“he looked”), the second that of understanding: he saw that they were standing in one spot, and so understood that they had no desire to cause him any trouble ... he, therefore, took the initiative and ran towards them.
(Silbermann translation courtesy of sefaria.org)