In their 2018 report on alleged electoral fraud in the 2017 General Election, the Electoral Commission said this,
"Allegations of double voting
2.20 In the days following the June 2017 UK general election, a number of people on social media claimed they had voted twice at the general election.
We received a significant amount of correspondence referring to these social media posts – our report on electoral registration at the 2017 UKPGE noted that at the time of publication we had received 1,013 emails and 15 telephone calls from members of the public referring to media reports about these
claims.
2.21 To date, we have received 60 letters from 47 MPs raising constituents’ concerns on this issue.
2.22 The National Police Chiefs’ Council (NPCC) has collated information from police forces across the UK about the current status of their
investigations into allegations of double voting. In December 2017 we reported that a total of 17 individual allegations of double voting had been
identified from claims made directly to the police or indirectly via elected representatives, the press or social media.
2.23 By the end of January 2018, we had received notification of five allegations of electoral fraud which related to double voting from police
SPOCs.
2.24 As detailed above one of these cases resulted in a conviction. Two cases resulted in no further action following the conclusion of police
investigations; one was classed as ‘other’ and deemed not in the public interest to prosecute; and the remaining case is still under investigation."