defragment SQL server database
How can I avoid that
  


The only way to avoid this is by not shrinking that is why shrinking is such a evil operation. But there are situations where conditions demand a shrink to be done to release excess space and for that case you have no option but to do it. You would then have to run defragmentation of the indexes. Paul Randal in This Blog showed alternate way (read the whole blog this is exactly what you may need).

Further I would also suggest you to read Mitigating Index Fragmentation


  The method I like to recommend is as follows:
  
  
  Create a new filegroup
  Move all affected tables and indexes into the new filegroup using the CREATE INDEX … WITH (DROP_EXISTING = ON) ON syntax, to move the
  tables and remove fragmentation from them at the same time
  Drop the old filegroup that you were going to shrink anyway (or shrink it way down if its the primary filegroup)
  


This may not always be possible in all environments but this is good alternate of shrinking.


  Or is there a way to defragment, for instance, if I restore a database, will it be defragmented again? Or will it have same fragmentation as the original db?


If you restore a database which has index defragmnated you do not need to run index defragmentation again after the restore. Restore creates exact copy of the database