完全没有区别,可互换,字典为证：
dull
1 dull duller dullest
词义：
If you describe someone or something as dull,you mean they are not interesting or exciting.
例句：
They are both nice people but can be rather dull.
I felt she found me boring and dull.
The documentary lasts for more than two-and-a-half hours,and there is scarcely a dull minute.
ADJ-GRADED disapproval
同义词：
= boring
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boring
词义：
Someone or something boring is so dull and uninteresting that they make people tired and impatient.
例句：
Not only are mothers not paid but also most of their boring or difficult work is unnoticed.
...boring television programs.
ADJ-GRADED
= dull,tedious