If propagation changes during your contact and you notice increasing interference from other stations on the same frequency, what should you do?

Radio wave propagation can change suddenly, causing stations that previously could not hear each other to begin overlapping and interfering. In this scenario, nobody is at fault, and neither party has exclusive rights to the frequency. The most practical and courteous solution is to simply move your contact to a clear frequency. Asking other stations to move because you feel you were 'there first' is poor etiquette, as they likely could not hear you when they started transmitting either. Increasing your output power will only exacerbate the interference for everyone else on the band.