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Currently, Sailor Moon Crystal (ONA 2014) is episodes 1-13, and Sailor Moon Crystal (ONA 2015) is episodes 14-26.

However, according to Viz's blog, the Sailor Moon Crystal Set 1 - Dark Kingdom Arc is episodes 1-14.

In light of this, and given that episode 14 is both the ending of the Dark Kingdom arc, and the beginning of the Black Moon arc (as both Metallia is defeated and Chibi-Usa appears in it), there are really 3 options:

Option 1: Leave episode 14 as part of ONA 2015. The problem with this is that that means the release will be on both the ONA 2014 and ONA 2015 page, since it's got episode 14 in it.
Option 2: Move episode 14 from ONA 2015 to ONA 2014. This way it'll match the official US releases.
Option 3: List episode 14 as part of both ONA 2014 and ONA 2015. This way, it's acknowledged as the last episode of the Dark Kingdom Arc and the first episode of the Black Moon Arc.

My suggest ion would be either Option 2 or Option 3. Option 3 does make it confusing, since it would look like the series as a whole is 27 episodes, and does still have the double release listing, but it's most accurate story-wise. Option 2, though, will match the official release.

I've not seen any of it or been following news about it so I'm coming in completely blind.
TL;DR: it's confusing and I have no current opinion. I know we've had a discussion about where episode divisions are different depending on the source, but I haven't turned it up yet.

According to the encyclopedia:
ep 13 aired 2015-01-03
ep 14 aired 2015-01-17

In the news we have:
"Starting on January 17, the second season includes the story from the original manga's 'Black Moon' arc" [source]
"The 'Animate Girls Festival 2014' event confirmed again on Saturday that the Pretty Guardian Sailor Moon Crystal television anime is continuing into its second season. (Toei Animation had confirmed in April that the anime will last 26 episodes, and the second season will be the second half of those 26 episodes.)" [source]
"Starting on January 17, the second season includes the story from the original manga's 'Black Moon' arc" [source]

Viz's streaming site splits the episodes up as 1-12, 13-24 and 25+

Hulu, however, has episodes 1-14 in Season 1, 15-26 in season 2 and 27+ in Season 3.

Episodes 13&14 were on the same disc in Japan.

--edit--

Relevant: animenewsnetwork.com/bbs/phpBB2/viewtopic.php?t=2940134

My preference is option 2. The way Hulu is organizing the seasons, and Viz is going to release it, makes the most sense. The first story arc is definitely incomplete without episode 14. That episode does start another story arc, but doesn't get that deep into it. So it's much more awkward for the split to be between 13 and 14 than between 14 and 15.