פירוש על קידושין 118:16

Rav does not know whether the first betrother intended his statement as a stipulation, “If I do not change my mind within thirty days, you will be betrothed to me from now” or a retraction, i.e. “No you are not betrothed to me until after thirty days.” If it was a stipulation, then she would be married to the first man. If a retraction, then she is married to the second because the first kiddushin was never valid. Therefore, she is doubtfully betrothed to both men and will require a get from both.
Shmuel holds that the statement was a stipulation. She is doubtfully betrothed for the first thirty days but after that the second person’s betrothal is removed because the first person’s takes effect.