Im still wondering what is so appealing about recording something you've already watched. You haven't really given a reason why this is so important for you.
Most people use timeshift either to be able to pause live TV (phone ringing, people at the door, urgent "business"), or to be able to rewind a few minutes to watch something again.
And I think a lot of people will become nervous if someone else can watch back the last 90 minutes of what they have watched.
Not what you watchet but what you missed as you have wrote 'phone ringing, people at the door, urgent "business"'. People doing many things watching tv and sometimes even 90 minutes is to short time to rewind.
What if someone changed channel during your urgent "business". Timeshift should recording in background all the time and an user shouldn't has in mind to remember to start recording.