I wasn't able to find a 2004 functional archive of the cartoonnetwork.com website, but they are considered as 2 separate series by DC Comics:

• Justice League
• Justice League Unlimited

Even if it's not a formal proof, this 2006 DVD seems to indicate that the numbering of the seasons have been restarted from 1 for Justice League Unlimited, and that, as such, it should be considered as a separate series.

It's hard to explain. And, perhaps I should have just reported the other series. I would personally consider Justice League + Justice League Unlimited as the "Complete Series", as it originally was. But, because of the mid-series rebranding that DC/WarnerBros occasionally does (also see Young Justice vs. Young Justice: Outsiders); some may know the series as two separate ones. I would, at the least, add seasons three-five of Justice League, not to confuse the OG fans. (They also rebranded the first three-parted episodes as a movie called Justice League: Secret Origins - it was all for marketing).

@AceMC said:

It's hard to explain. And, perhaps I should have just reported the other series. I would personally consider Justice League + Justice League Unlimited as the "Complete Series", as it originally was. But, because of the mid-series rebranding that DC/WarnerBros occasionally does (also see Young Justice vs. Young Justice: Outsiders); some may know the series as two separate ones.

As per our rules, we should list the series as they were originally broadcasted, even if they have created 2 separate series just for marketing purposes. Taking in account that they have restarted the seasons numbering at one, we need to consider them as 2 separate series.

To take a more recent example, Dexter: New Blood is the continuation of the Dexter story, but the network has marketed it as a separate series and , as such, we should list it as a separate series.

We always list the series as they were originally broadcasted.

I would, at the least, add seasons three-five of Justice League, not to confuse the OG fans.

Duplicates aren't allowed.

Dexter: New Blood is a sequel to Dexter. As I've explained; Justice League Unlimited is just Season 3, 4, and 5 of Justice League. Choose to leave it incorrect if you wish. But, even Rotten Tomatoes has it ordered properly, as well.

We match the original broadcast. They were broadcasted as Justice League Unlimited Seasons 1, 2 and 3 so we should list them this way.

Exactly the same as the new Dexter season is broadcasted as Dexter: New Blood season 1 and not simply as a new season of the Dexter series.

There are no differences between the 2 cases.