這兩個to的意思分別是甚麼？

第一個 to 是 to varying degrees，形容 impacts (衝擊、影響) 逹到不同情度。

第二個 to 是 to Hong Kong people，Hong Kong people 是動詞 caused 的間接受詞 (indirect object)，直接受詞 (direct object) 是 impacts。當間接受詞放於直接受詞之後，間接受詞前就要加 to。

They have caused positive and negative impacts to Hong Kong people.

當間接受詞放於直接受詞之前，間接受詞前就無須加 to。
The have caused Hong Kong people positive and negative impacts.

I gave a book to Mary. (direct, indirect objects)
I gave Mary a book. (indirect, direct objects)

2012-10-19 00:46:57 補充：
第一個 to 除 degree 外，還常用於 extent

to certain extent
to a great extent