Sen. Bernie Sanders (I-VT) speaks during a press conference at the U.S. Capitol January 30, 2019 in Washington, DC.

Sen. Bernie Sanders, the Vermont independent who is seeking the Democratic presidential nomination, for years has drawn a pension for his eight-year stint in the 1980s as mayor of Burlington even has he received a salary as a member of Congress.

Sanders, who earns $174,000 as a senator, received $5,241 from Burlington's pension system in 2018, according to his federal income tax return.

His total income with his wife, Jane O'Meara Sanders, that year was $561,293, which was down from the more than $1 million they earned in the prior two years, largely as a result of his book about running for president in 2016.

Public financial disclosure records show that Sanders, who began serving in the U.S. House of Representatives in 1991 and in the Senate in 2007, has received nearly $62,000 in Burlington pension payouts since 2005.

That practice, known as "double-dipping," isn't illegal.

Nor is it unknown in Congress, where a National Journal study in 2013 found that nearly 20% of members have drawn government pensions while serving in the Senate or the House, for a total haul of more than $3.6 million in such pensions in the prior year.

But double-dipping does rub some public watchdogs — and even elected officials — the wrong way.

In fact, in 2014, former Obama administration official Ro Khanna, while conducting a challenge to incumbent Rep. Mike Honda of California, blasted his fellow Democrat Honda for double dipping. Honda at the time was receiving three pensions related to government jobs, including one belonging to his late schoolteacher wife.

"Those pension benefits are supposed to support public workers in their retirements — not while they're still serving and collecting a salary of $174,000 a year," Khanna said in 2014.