const uint8_t MAX_ADDRESSES = 3;
String knownAddr[MAX_ADDRESSES];
uint8_t knownAddrIndex = 0;

bool isAddressKnown( String address )
{
  if ( knownAddrIndex != 0 )
  {
    for ( uint8_t i = 0; i < knownAddrIndex; i++ )
    {
      if ( knownAddr[i] == address )
      {
        return true;
      }
    }
  }

  return false;
}

bool addAddress( String address )
{
  Serial.print( "Adding address \"" );
  Serial.print( address );
  Serial.print( "\" : " );

  if ( isAddressKnown( address ) )
  {
    Serial.println( "fail, already known" );
    return false;
  }

  if ( knownAddrIndex == MAX_ADDRESSES )
  {
    Serial.println( "fail, array is full" );
    return false;
  }

  Serial.println( "success" );
  knownAddr[ knownAddrIndex++ ] = address;
  return true;
}

void setup()
{
  Serial.begin( 115200 );

  addAddress( "address1" );
  addAddress( "address1" );
  addAddress( "address2" );
  addAddress( "address2" );
  addAddress( "address3" );
  addAddress( "address4" );
}

void loop()
{
}