How to make a teleport command that does not work in the Nether?

The Dimension parameter does not exist as a target selector argument, but is present in the entity data.

Sadly, /tp does not allow you to check for entity data, so you will have to create a workaround using a scoreboard objective.

/scoreboard objectives add inNether dummy

One the objective exists, create a fill/setblock clock and run the following two commands, in this order:

/scoreboard players set @a inNether 0
/scoreboard players set @a inNether 1 {Dimension:-1}

This will set the objective to 1 for everyone in the Nether and you can use this to limit your tp command to

/tp @a[score_tpHome_min=1,score_inNether=0] 68 68 68

That being said, this would allow the use of the TP in the End, which almost definitely results in death. I suggest turning the logic around by doing

/scoreboard players set @a inOverworld 0
/scoreboard players set @a inOverworld 1 {Dimension:0}

instead, and check for score_inOverworld_min=1.

This is almost a version of @MrLemon's answer. Run the following command once:

/scoreboard objectives add Dimension dummy

Put the following commands on a fill/fast clock:

/scoreboard players set @a Dimension -2 {Dimension:-2}
/scoreboard players set @a Dimension -1 {Dimension:-1}
/scoreboard players set @a Dimension 0 {Dimension:0}    

Then do the tp as following:

/tp @a[score_tpHome_min=1,score_Dimension_min=0] 68 68 68

How this works is it gets the player dimension by setting the scoreboard objective Dimension to the dimension that they are in.