Generally speaking, I consider it much more common accidentally to omit one of two similar words or phrases in a text (haplography) than to accidentally repeat a phrase (dittography). This is due in part to the fact that mentally engaged scribes are likely to note the repetitive copying of dittography and correct their errors, whereas scribes commiting haplography are more likely to be left unaware of their errors. In reading ancient manuscripts, this seems to bear out statistically. I have been reminded recently, however, about the real possibility of dittography in my reading of the Great Isaiah Scroll. I have recently come across two clear instances of dittography in 1QIsa(a) which serve as good reminders.
MT: ...שובו לאשר העמיקו
"Return to him whom the sons of Israel have so greatly rebelled against."
1QIsa(a): שוביו לאשר לאשר העמיקו
The Isaiah Scroll here has accidentally repeated the word לאשר translated "to him whom," rendering nonsense.
MT: בצלה אך שם
"... in its shadow. Also there ..."
1QIsa(a): בצלה אך אך שמה
The Isaiah Scroll here has accidentally repeated the word אך translated "also," rendering nonsense.