Jonah 3:1
Now the word of the Lord came to Jonah the second time, saying…

I have a saying – if God says it once He means it. If He says it twice He really means it.
The Bible has many examples of God repeating Himself just to get the point across to us.
For instance, it is important for a believer to live a holy life.
How do I know?
God said it in Lev 11:44 – you shall be holy; for I am holy.
That one time should do it.
But we don’t quite get the importance of it so God repeated Himself.
Lev 11:45 You shall therefore be holy, for I am holy.
Lev 19:2 ‘You shall be holy, for I the Lord your God am holy.
Lev 20:26 you shall be holy to Me, for I the Lord am holy
1 Peter 1:16 “Be holy, for I am holy.”
That’s just one example!
So when God told Jonah to go to Nineveh, He was serious about the mission.
When God told Jonah the second time, Jonah did as he was told.
If God is telling you something don’t make Him tell you twice!