It is hard to say that from what time or for what reason, they became deadly rivals.

很难说出从哪个具体的时间或事件开始，他俩成了死对头。

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There was a sort of friendship between the two men, who were all the while deadly rivals.

这两个男的素来是冤家对头，如今竟有了一种友情。

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Bombardier and Brazil's Embraer are deadly rivals in the fast-growing regional jet market, which involves commercial planes with less than 100 seats.

BBC: Americas jet trade row escalates

After recent deadly fighting between the Shiite rivals Mehdi Army and Badr Organization last week in Karbala, Baghdad, and Babil province, al-Sadr announced he was suspending the militia operations and restructuring the group.

CNN: 26,000 U.S., Iraqi troops conduct offensive