Yes, it’s a problem. The traditional way of thinking about trials is that they should be independent and not incorporate information that we already have.
Scott Berry talks about this issue (among others) here
also this datamethods thread
Scott makes the point that when a 1000 patient trial is suggestive but inconclusive (“fails to show significance”) the usual reaction is to do a 2000 patient trial, rather than add maybe a few hundred to the existing evidence. At best this is a waste of resources.