Minecraft – ny way to teleport someone to the nether? (1.14)

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.

You could use a setblock command, using a redstone block, instead of teleporting the player right away. The block would be placed next to some redstone, which would trigger a command block to say "teleporting", and run to a line of repeaters to create delay, which is what I assume you want. At the end, have it trigger two command blocks: One to teleport the player, the other to remove the redstone block, to ensure that this would work more than once. Here's a simple schematic:

     C2
C1   ^
R>>>>+>>C3

Where R is where the redstone block would be placed, C1 is the command block that would have the command to tell the player, C2 is the command block that destroys the redstone block, C3 is the command block that actually teleports the player. >> being a right-facing repeater, ^ being an up-facing repeater and + being redstone dust. You could also take advantage of the Scoreboard system so the command block teleports the correct player.