You should draft Turner in the same spot regardless of whether or not you have LT already on your team or not. You should draft Turner when he provides more value to you than any other players available, and that point should be the same regardless of if you have LT or not.
In my opinion, which players you already have on your roster can substantially affect the value to your team of other players you're considering drafting.To take an obvious example, if you draft Peyton Manning in the second round, that drops the value to your team of taking Carson Palmer in the third (assuming you start one QB). The value of any player you draft is roughly proportional to both the value he'll give you when he starts, and the chance that he'll start for you. Palmer has low value once you've drafted Manning because there's not much of a chance he'll start for you.
With Turner, there's not much of a chance he'll start for your team, but you're drafting him in the eight or ninth round. There's not much of a chance
any player at a key position you draft there will start for you. But if Tomlinson gets hurt, Turner will both (a) score lots of fantasy points, and (b) fill a major team need for you. Part (b) is why Turner is more valuable to a Tomlinson-owner than to a non-Tomlinson-owner.
Or, as I put it
here:
Handcuffing refers to the practice of drafting two players at the same position from the same NFL team. Suppose I draft Jacksonville Jaguars RB Maurice Jones-Drew in the second round. I might then "handcuff" Jacksonville Jaguars RB Fred Taylor to Jones-Drew by picking Taylor in the sixth round. Since Jones-Drew's playing time will be negatively correlated with Taylor's playing time, and I already have Jones-Drew on my team, Taylor will be worth more to me than his VBD value would indicate.
Here's why. Fred Taylor has direct value to my fantasy team when he starts for my fantasy team (rather than when he is on the bench). When is Taylor likely to start for my fantasy team? When both of the following occur: (a) one of my starting RBs is injured or is performing poorly, and (b) Taylor starts playing a lot for the Jaguars.
Situations (a) and (b) are more likely to coincide for me if I own Jones-Drew than if I do not own Jones-Drew. (Do you see why?) Therefore, Taylor is more likely to have value for me if I own Jones-Drew than if I do not own Jones-Drew.