Are you talking about someone doing formal twice? or someone who's done formal and is now doing informal?
At my alma mater, we had COB in the fall and formal in the spring. Many times, we had WAY more girls COBing than we had spots for (read: one year we had 21 girls wanting 5 spots!) and we were excited when girls decided to go through formal rush because they wanted to be in our chapter (only upperclassmen could do COB so if they didn't do formal rush, they risked having to COB as a junior). There were also girls who went through formal twice, having dropped the 1st time before prefs (or just after...before getting a bid) and we took a few like that. Sometimes, however, let's just say some girls seemed a little 'desperate' and would do formal and drop out or get dropped, then try to COB, and then try formal again.
Let's just say that wasn't considered a good thing.
It all depends on the girl. If she's genuinely interested, give her a shot. Also remember that people change over time and this girl may not be the same girl she was the year before. Always give girls the benefit of the doubt first. Some girls will tell you why they are rushing again, but don't push them to disclose. There will always be girls who aren't liked by any chapter who continue to rush and rush and rush...but there are those who rush twice who make great assets to chapters!
That's my two cents (or three or four
)