Puppet - How to deal with a Duplicate Resource Declaration error

01 same named resource.pp

# ORIGINAL ROLES
class role::web {
  # snip ...
  
  sysctl { "net.core.somaxconn":
    # snip ...
    value => 8192
  }
  
  # snip ...
}

class role::redis {
  # snip ...
  
  sysctl { "net.core.somaxconn":
    # snip ..
    value => 1024
  }
  
  # snip ...
}

# When you try to include both of these classes in a node (that plays both roles, for instance), you're going to have problems: 
#   Error: Duplicate declaration: ______ (whatever resource) is already declared in file ...

02 define based virtual resource.pp

# defines.pp
define base::sysctl::somaxconn ($value=undef) {
  if $value {
    $ensure = 'present'
  } else {
    $ensure = 'absent'
  }

  sysctl{ "net.core.somaxconn":
    ensure => $ensure,
    permanent => 'yes',
    value => $value
  }
}

# NEW ROLES
class role::web {
  # snip ...
  
  @base::sysctl::somaxconn { 'web': value => 8192 }  

  # snip ...
}

class role::redis {
  # snip ...
  
  @base::sysctl::somaxconn { 'redis': value => 1024 }  
  
  # snip ...
}

# nodes.pp
node 'web1' {
  include role::web
  realize Base::Sysctl::Somaxconn['web']
}

node 'redis1' {
  incldue role::redis
  realize Base::Sysctl::Somaxconn['redis']
}

node 'web_and_redis1' {
  include role::redis
  include role::web
  
  realize Base::Sysctl::Somaxconn['web']
  # You can also "pick" the redis role's somaxconn by passing the
  # title of the resource defined by the redis role in the call to
  # `realize` above. That happens to be 'redis' in this case.
}