Sounds like Counterfeit by Alan E. Nourse, first published in the August 1952 edition of Thrilling Wonder Stories. The story is also known by the alternate title of The Counterfeit Man.
From the Wikipedia summary:
"The Counterfeit Man". The medical officer of an exploratory spaceship returning from Ganymede determines the crew has been infiltrated by at least one highly malicious shapeshifting alien. He attempts to force the intruders, who can almost perfectly mimic human physiology down to the cellular level, to betray themselves. Succeeding with one, he ejects it into space, gaining the cooperation of the expedition commander. Upon arrival at Earth, the doctor sabotages the ship to temporarily strand it in orbit, going ahead in a shuttle - with proof in hand - to order a quarantine. When the ship lands, the entire crew is however accounted for; the incredulous doctor storms back into the ship to search it himself, only to be ambushed and killed by the remaining alien, who has taken on his own guise and who, emerging, heads into the surrounding city. This story was adapted as the second episode of the BBC television series Out of the Unknown.
You can read the full story at the Internet Archive, but here are some relevant extracts:
Crawford waved the grey cards at him. "It's right here." he said. "These are lab reports. As you know, I ordered a complete examination on every man aboard, the day after we blasted from Venus. A normal procedure—we had to be sure that nothing had been picked up by the exploring parties, or anybody else. Among other things,. we ran complete lab studies on each of the men—urine, blood chemistry, so forth. We got every man on board into the lab within two days after blasting, and took blood samples from them. And we got some remarkable results.”
“There are eighty-one men on the ship,” the doctor continued. “Of these, eighty presented a clean bill of health, absolutely negative reports on everything. But one man was slightly different,” He tapped the cards slowly with a slender finger. “One man showed everything normal—blood count, chlorides, calcium, albumin-globulin ration — everything just the way it should be. Then we ran his blood sugar.” The doctor streatched his leg, regarding his toe closely. “This man didn’t have any blood sugar,” he said quietly. “Not a trace.”
Like a cat the doctor clamped down the hatch lock, and savagely pressed a stud on the wall. A red light went on in the chamber, and the exhaust machinery whirred into motion. Wescott looked up suddenly, eyes wide with alarm, and he sprang to his feet. “Doc!” he shouted, his voice coming thin and brittle through the panel. “Doc! Throw that switch! I haven’t got a suit on—” Jaffe’s breath hissed, and he stared at the doctor, paralyzed. “What are you doing?” he yelped. “You’ll kill him.” “Just watch,” growled the doctor. The man in the chamber was standing tense now, terror in his face. “Doc!” he shouted, desperately. “Doc! Turn it off. Stop it, Doc, stop it!” His eyes were wide with fear, and his face twisted into a grimace of sheer, impotent, screaming rage. “Stop it, stop it, I’m chocking—” He banged at the hatch with his fist until the blood ran and smeared on the hatch—and became something different from blood. His hands went to his throat, and he sank to his knees, as the pressure gauge w-ent down and down, and he twisted on the deck, coughing. Suddenly red blood poured from his nostrils, and he convulsed on the floor, and lay still.
With a scream he whirled on the huge, gaunt figure standing in the door, saw the black hair and the hollow eyes, the long thin jaw with the black stubble, the slow, easy smile spreading across the full lips—
He screamed, again and again, as he backed away, eyes wild with fear. He screamed, and he knew as he screamed that nobody could hear him.
He was staring—into his own face.
The Gantry crane creaked its weariness to the world as the platform moved lower and lower, and Dr. Crawford stepped to the ground. He grinned at the Police Commander, and rubbed his stubbled chin. “I’m for home and a shave,” he said. “I’ll be back tomorrow for the final disposition of the records. Better not let anything be disturbed until then.” The Commander nodded and turned back to the field desk.
The doctor walked slowly up the ramp to the Spaceport Building, through the lobby, and out onto the street. There he paused, feeling his feet turning almost instinctively toward the Coral Street subway.
But he didn’t start for the Coral Street subway, to take him out to the suburbs, to his home, to his wife.
Instead, with a curious eager brightness in his eyes, he turned onto the downtown thoroughfare, and vanished into the crowds of people toward the heart of the city. . . .
