title: wg-quick: `wg0' already exists when start wireguard in CentOS 7.6
body: <p>when I am using this command to start wireguard in CentOS 7.6:</p>
  <pre><code>wg-quick up wg0
  </code></pre>
  <p>shows this error:</p>
  <pre><code>wg-quick: `wg0' already exists
  </code></pre>
  <p>I am using <code>ifconfig</code> command to see the wg0:</p>
  <pre><code>wg0: flags=209&lt;UP,POINTOPOINT,RUNNING,NOARP&gt;  mtu 1420
          inet 10.0.0.1  netmask 255.255.255.0  destination 10.0.0.1
          unspec 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00  txqueuelen 1000  (UNSPEC)
          RX packets 0  bytes 0 (0.0 B)
          RX errors 0  dropped 0  overruns 0  frame 0
          TX packets 0  bytes 0 (0.0 B)
          TX errors 13  dropped 0 overruns 0  carrier 0  collisions 0
  </code></pre>
  <p>this is my config:</p>
  <pre><code>[root@izbp19pke6x0v6ruecuy1yz wireguard]# cat wg0.conf
  [Interface]
  PrivateKey = [redacted]
  Address = 10.0.0.1/24
  PostUp   = iptables -A FORWARD -i wg0 -j ACCEPT; iptables -A FORWARD -o wg0 -j ACCEPT; iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
  PostDown = iptables -D FORWARD -i wg0 -j ACCEPT; iptables -D FORWARD -o wg0 -j ACCEPT; iptables -t nat -D POSTROUTING -o eth0 -j MASQUERADE
  ListenPort = 7456
  
  [Peer]
  PublicKey = Cfrr58B8PjtpZhJzqdFXwpXQOnZedQpOOWjrkeM7p30=
  AllowedIPs = 10.0.0.3/24
  
  [Peer]
  PublicKey = b3rP/aN53ZY7IwzOWyxfxLEw7atwS2GpFe0kLchHhk4=
  AllowedIPs = 10.0.0.2/32
  </code></pre>
  <p>but the process not exists:</p>
  <pre><code>[root@izbp19pke6x0v6ruecuy1yz wireguard]# lsof -i:7456
  [root@izbp19pke6x0v6ruecuy1yz wireguard]#
  </code></pre>
  <p>what is the problem and what should I do to fix it?</p>