From Forbes:
While it might seem far-fetched that colleges will communicate with each other, it is a real possibility. In 2016, Katharine Fretwell, dean of admission and financial aid at Amherst College, said that her college and about 30 other colleges, share lists of students admitted through early decision. She said that she is likely also to share names of students who were accepted during ED, but choose not to attend to due to financial aid or other reasons. Currently, the Justice Department is investigating this issue of colleges sharing information.