I had to create an EFI partition to install ubuntu 18.04. Now I want to create another boot on the same HD with 18.04 again. Do I need to create another EFI partition?
You need only one EFI partition for a computer in general.
No matter how many OS you have.
You may need multiple EFI partitions only in some special cases when you swap disks, change boot device in UEFI, etc.