If you want a line of dialogue to only ever play once, you could have a variable in the condition. You check for false, and set to true, so after the first time it plays it can never fill the condition (and so never plays again). At least, that's my trick for those one-time intro bits I do.
So it would look a little like this:
all:"CLEAR"
dialogue_name:"example"
initial_settings:{"varName":"false","otherVar":"false"}
CUSTOM:
intro:"THIS ONLY PLAYS ONCE"{"check":{"varName":"false","set":{"varName":"true"}}
intro:"THIS PLAYS ANY AMOUNT OF TIMES"
intro:"AND SO DOES THIS"
You could also use one of the built-in variables like *FINISHES* to have multiple lines that each play on a specific restart, which is detailed in one of the first few posts of this thread.