In case of inserted a fixed value into autoincrement how to automatically skip into the next sequence value without causing insert to have an error?
You can use the following statement to synchronize the sequence with the current max value of the ID column:

select setval(pg_get_serial_sequence('dummy', 'id'), (select max(id) from dummy));


Online example: https://rextester.com/GMK2630

You can run this manually after you have inserted rows with explicitly supplied values. 



Note that this can still result in a wrong sequence value if this is done concurrently from multiple transactions because a transaction does not see the values inserted (but not committed) by other transactions. 

But I would strong recommend to simply let the default do its job, adjusting the sequence like that is more a hack, rather than good coding practice.