Add check if rule name already exists in /autolink add command

[dmarushkin]

Thare are no checks if Name already exists in function executeAdd:

server/autolinkplugin/command.go:
`
func executeAdd(p *Plugin, c *plugin.Context, header *model.CommandArgs, args ...string) *model.CommandResponse {
if len(args) > 1 {
return responsef(helpText)
}
name := ""
if len(args) == 1 {
name = args[0]
}

err := saveConfigLinks(p, append(p.getConfig().Links, autolink.Autolink{
	Name: name,
}))
if err != nil {
	return responsef(err.Error())
}

if name == "" {
	return executeList(p, c, header)
}
return executeList(p, c, header, name)

}
`

After adding second rule with the same name we can't change prev or new rule or even delete new one. Instead we got this message:

"Visa" matched more than one link: ["Visa" "Visa"]

So we can fix that by checking name in existing rules.

[bradwood]

I have fallen foul of this bug too and, what makes it annoying is if you try to delete the duplicates with a slash-command you cannot.