Remove duplicate user sessions during login using Sitecore PowerShell Extensions. Solution for https://github.com/SitecoreSupport/Sitecore.Support.94909.

Remove-Multiple-Sessions.ps1

# Solution for https://github.com/SitecoreSupport/Sitecore.Support.94909
# Cannot login in Sitecore CMS due to multiple same users already logged in
# Saved to /sitecore/system/Modules/PowerShell/Script Library/[CUSTOM_MODULE_NAME]/Pipelines/LoggedIn/Remove Multiple Sessions
$pipelineArgs = Get-Variable -Name pipelineArgs -ValueOnly
$username = $pipelineArgs.UserName

if(!$username) { return }

$sessionId = [System.Web.HttpContext]::Current.Session.SessionID
Write-Log "The current session Id is $($sessionId) for user $($username)"

$availableSessions = Get-Session | Where-Object { $_.Username -eq $username}
foreach($session in $availableSessions) {
    if($session.SessionId -eq $sessionId) { continue }
    Write-Log "Removing duplicate session $($session.SessionId) for user $($username)"
    $session | Remove-Session
}