It was a "weekend of hell" for a son and mother after she received two injections of a coronavirus vaccine.
Barbara Wright, from Truro, who has vascular dementia received a dose on Thursday and another on Saturday.
Her son, Graham, was worried what the two doses would do, but said his mother seemed "her normal self".
A governing body member for NHS Kernow in Cornwall said: "We apologise to the family that this incident has occurred."
Mr Wright contacted the emergency services after his elderly mother, 89, had received the vaccine at her home on both 21 and 23 January.
"They'd [a paramedic] never heard of anybody having two Covid vaccines within such a short period and they didn't really know what to do," he told the BBC.
He has shared the story as he does "not want this to happen to anybody else".
Rob White, NHS Kernow's clinical lead for the Covid-19 vaccination programme, said receiving two doses in a short space of time "will not cause the patient any harm" and they were investigating what had happened.
The UK's chief medical officer has said the second dose of the coronavirus vaccine will be given up to 12 weeks after the first dose.
Mr Wright said a nurse and a doctor from the same practice had both injected his mother.
A doctor from the practice apologised to him and blamed the mistake on a "computer glitch", he explained.
Mr Wright said he did "feel sorry for the doctors" and could understand mistakes being made, but felt a medical practice blaming a glitch was "not the answer".
"It makes me feel I can't trust them," he added.
The medical practice has been approached for a comment.