Fidelity Investments said an undisclosed number of brokerage clients had online orders delayed or accidentally duplicated yesterday.

The malfunction lasted for about 30 minutes beginning at about 9:45 a.m. New York time, Stephen Austin, a spokesman for Boston-based Fidelity, said in a telephone interview. Fidelity will reimburse any clients who lost money as a result of the error, he said.

“We took prompt steps to resolve this and the site is now performing normally,” Austin said.

Fidelity oversees $4.5 trillion as a record-keeper for investors in retirement accounts, for financial advisors served by the firm and for individual customers of its online brokerage business. The malfunction comes two months after E*Trade Financial Corp. said some customers were briefly unable to access their brokerage accounts on Jan. 8, locking them out of the market as comments from the Federal Reserve spurred a surge in trading. Charles Schwab Corp. was targeted by a cyberattack in April that disrupted access to its service.

Zachary Prensky, an analyst with Little Bear Investment LLC in New York, said he placed an order to short-sell the SPDR S&P 500 ETF Trust at 10:02 a.m., which Fidelity filled twice, 20 minutes apart. Prensky made an extra $1,000 on the mistake when the ETF declined, he said in a telephone interview. Investors benefit from a short sale when a security falls in value.

Fidelity had 19.2 million brokerage accounts and processed an average of 407,400 “commissionable trades” every day during the fourth quarter of 2013, according to the firm’s website. Fidelity also offers broker-dealers clearing and execution services.