Loading an old game is just another way to start a game, so you have to do all the things necessary for a normal game. That includes telling the game server who the other players are.
Now, this process is unfortunately not optimized for the most common case where you want to just be in the shoes of one player and have the other players taken over by AIs. In addition, Mic seems to have used a single player game (only one player), which has often problems loading. So I made a new, two player game (10908430) to illustrate the issue.
Go to "New Table", press "Load Old Game", then enter the number 10908430 and press "Load Game". This you already did with Mic's game.
Now, press "Add bot" once. This will add Lord Rattington to your table. Since the original game had 2 players, this is enough, the "Add bot" button will go away.
Now press "Player Order". This is where it is determined which of the current players (you and Lord Rattington) take the shoes of which original player (Ingix (me) and Lord Rattington). This could do with some reasonable defaults, but for now click on the checkmark near "Random" to make the order not random but chosen by you.
A small window will pop up, which should on the right side have the heading "Original Player", with Ingix on top and Lord Rattington beneath. On the left side are the current players, which should be your name on top and Lord Rattington beneath. If that's the case, you can hit "Done", because that is what you want: You take my position and Lord Rattington plays Lord Rattington.
If for whatever reason the order is reversed, you can use the small arrows to change the order of the players on the left side. Hit "Done" when you are ... done
Now press the button "Ready". This should take you into the game, Lord Rattington will do his turn 7 and you will start turn 8. Take a look at 'your' turn 7 in the log. I bought Laboratory, but gained the cheaper card first (Vassal), then gained the Laboratory and then gained the Duplicate of the Laboratory.
If you want to replay that yourself, type "/undo" into the chat window (don't type the quotes, but *do* type the slash). The log will change and add lots of small dots to the start of its lines. Each dot is a decision a player has made, like playing a card, choosing (or not) to Duplicate something, etc.
Click on the dot before the line "Mic buys a Laborarory" (Mic will be replaced by your in-game initials) on your turn 7. This will bring you to the point *before* that action. You have Haggler in play and $6 to spend. Duplicate is on your Tavern mat.
Buy a Laboratory (or whatever you fancy, in the following I describe what I did). This will trigger Haggler's effect, and the game asks you which cheaper card you want to gain. Choose Vassal. Now you gain the Vassal and, as you noticed, the game gives you the option to Duplicate the Vassal. Please also note that you didn't gain the Laboratory yet, the supply pile still shows that all 10 are present there, so Duplicate *can't* at the moment offer you to gain it.
Decline to Duplicate the Vassal by pressing the "Done Reacting" button. *Now* you finally gain the Laboratory (watch the animation, or check your discard pile afterwards, it shows a Laboratory on top now). Now you also get the option to Duplicate the Laboratory. You can do so if you want (by clicking in the log), or also decline (by pressing "Done Reacting" again).
The same way it would have played out in your original game. You thought you had missed the point where Duplicate should offer you to Duplicate your Harem, but that point hand't come yet!