lf you get the Ally Duke it adds +1 Maximum Sanity & discard to immediately restore your Sanity to its maximum.

Does this +1 Maximum Sanity last all game or untill you loose Duke, if its the latter & you discard Duke to gain maximum Sanity is the +1 taken into account or not.

If you start the game with Duke, you get 1 extra Sanity Token.

The increased maximum Sanity only lasts as long as you have Duke. Once you discard him, you also lose the +1 Maximum Sanity bonus, so you are returned to your original Sanity without the +1.

l never started with Duke, l got Duke from an Location Encounter.

There is an Investigator that starts with Duke.

If you gain Duke through an Encounter you do NOT get 1 Sanity Token right away. You have to find a way to gain a Sanity Token.

To further answer your first question:

Lets say you have Max Sanity 5 and Current Sanity 5.
Having Duke gives you MAX Sanity 6, but Current Sanity remains 5.
Now say you are down to Current Sanity 2 and Discard Duke, which brings you back to Max Sanity 5, and restores you to Current Sanity 5.

dkw has it right. Duke only grants temporary maximum sanity. He only increases your actual sanity if you start with him as an Ally during setup (Ashcan, Leo, or Kane are investigators that collect allies).

Also realize you can only discard him before or after effects, not during. So if an encounter woulod drop you below 1 sanity, you can't immediately discard duke to save your self. You would have to wait until you went insane and then discard him to regain the sanity.

Let me see if l understand this.

So if my Sanity is 5 & l have 2 remaining & aquire Duke as a Ally, if l pay $2 to have my Sanity increased to my Maximum then l would make it up to 6 so long as l have Duke, when l lose Duke my maximum would go down to 5 again, then if l had 6 Sanity at that time l would have to lose one Sanity.

ls this correct.

Yes, that is correct.

Thanks for all your help.