How to interpret duplication from MultiQC/FastQC?
I would agree with your statement.  Typically the ratio of unique to duplicated reads is a function of input library complexity (number of input molecules) verse sequencing depth (the amount of molecules sequenced).  

In the example you gave you have about a 2:1 ratio of duplicates to unique reads which suggests that these libraries have been sequenced to their fullest and additional sequencing would yield few new unique reads.