is 1 , and the hand has to be closed. You hold two identical sequences: the same three numbers in the same suit.
The name players use is iipeikou (一盃口). This site's glossary also lists it as Pure Double Sequence. The EMA 2016 rules call it pure double chow.
Same numbers, same suit
234 man twice is iipeikou. 234 man and 234 pin are two suits, so they don't count. One run in each of the three suits is sanshoku doujun, a different yaku.
The hand waits on 
or . Only one of them makes the yaku.
makes 234 man twice. That's iipeikou plus pinfu: 2 han 30 fu on a . A collects 2,000. The dealer collects 2,900.
makes 123 and 234. The numbers no longer match, so the hand is alone: 1 han 30 fu. A non-dealer ron collects 1,000. The dealer collects 1,500.
A tsumo on is 3 han 20 fu: pinfu, iipeikou, and menzen tsumo. A non-dealer collects 700 from each other non-dealer and 1,300 from the dealer.
The glossary entry is the short form: two identical sequences in the same suit, only.
Closed only
A , a , or an removes it. There is no open han for it. Tenhou's rules mark iipeikou as closed only. The EMA 2016 rules define it as a concealed hand with two identical chows. That matches the row on this site: 1 han closed, and nothing open.
Without pinfu
556677 pin is 567 twice. The hand waits on only, a , so it isn't pinfu. Iipeikou is the only yaku.
A ron is 1 han 40 fu: the base 20, plus 10 for a closed ron, plus 2 for the middle wait. 32 rounds up to 40. A non-dealer collects 1,300. The dealer collects 2,000. Those payments have no , no , and no sticks.
A closed ron is 30 fu only when the hand is also pinfu. Any triplet, a pair, or an edge, middle, or pushes it to 40 or more.
Two of them
Two iipeikou in one hand is ryanpeikou: 3 han, closed only. You don't add the 1 han iipeikou on . The EMA 2016 rules say so in the entry. Most ryanpeikou hands also read as , and chiitoitsu covers which reading to score.
Common mistakes
- Calling after you hold the two runs. The han drops to 0, even when both runs are still in your closed tiles.
- Counting 123 and 234 as a pair of runs. The three numbers have to match.
- Pairing runs from two suits. 234 man and 234 pin are not iipeikou.
- Pricing an iipeikou-only ron at 30 fu. Without pinfu, it's 40 fu or more.
What sits with it
Pinfu sits in the same hand when the pair is plain and the wait is two-sided. Tanyao sits in it when every tile is 2 through 8. Riichi fits too, because the hand is already closed.
Price an iipeikou hand
The score calculator turns 2 han 30 fu, iipeikou with pinfu, into 2,000 or 2,900. Without pinfu, a closed ron is 40 fu or more.
Open the score calculator