JPMorgan Chase & Co., Goldman Sachs Group Inc. and Morgan Stanley are extending their dominance in underwriting equity offerings — helped by sales of financial companies' shares, including their own.

The three banking companies together control 42% of the global market so far this year, according to data compiled by Bloomberg News. This is up from a 30.7% share for the three top underwriters in the first six months of 2008 and the biggest concentration for any first half in at least a decade.

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