Attempt to incrememnt counter in Sequel is creating additional table entries

I have a program which opens up a bunch of tars, looks for certain file names, and then adds those tars and filenames to a sqlite database. The same file names occur in multiple places within the tar, so I created a table which points to a unique tar/file tuple and contains a counter for how many times that tuple appears in the tar. Here is the code for the table creation:

DB = Sequel.sqlite('tars.db')

DB.create_table? :tars do 
  primary_key :id
  column :name, String, :unique=>true
end

DB.create_table? :pfiles do
  primary_key :id
  column :fname, String, :unique=>true
  column :nname, String, :unique=>true, :null=>true, :default=>nil
end

DB.create_table? :pfiles_tars do
  primary_key :id
  foreign_key :tar_id, :tars
  foreign_key :pfile_id, :pfiles
  unique [:tar_id, :pfile_id] 
  column :counter, Integer, :default => 1
end

class Tar < Sequel::Model
  many_to_many :pfiles
end

class Pfile < Sequel::Model
  many_to_many :tars
end

class Pfiles_Tar < Sequel::Model
end

At some point I create the tar entry and the file entry. Then I want to check if an entry exists in the joint table. If it does, increment the counter and create it if not. This is the code that I thought should do this:

sql_tar = Tar.find_or_create(:name => tfile)
sql_pfile = Pfile.find_or_create(:fname => f)

if Pfiles_Tar.find(:tar_id => sql_tar[:id], :pfile_id => sql_pfile[:id])
  Pfiles_Tar.find(:tar_id => sql_tar[:id], :pfile_id => sql_pfile[:id]).update(:counter => Sequel.expr(1) + :counter)
else
  Pfiles_Tar.create(:tar_id => sql_tar[:id], :pfile_id => sql_pfile[:id])
end

But for some reason this doubles the number of table entries I have in the joint table. If I change the else statement to find_or_create instead of just create

Pfiles_Tar.find_or_create(:tar_id => sql_tar[:id], :pfile_id => sql_pfile[:id])

then I get the correct number, but I fail to see why. Can anyone explain what I am doing wrong? I am pretty new to both Ruby and Sequel.