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will md5 sum remain same on a topic ?

yes.

or does it change with each message although they are of same type?

The MD5 of a message is calculated as follows (from wiki/ROS/Technical Overview - Message serialization and msg MD5 sums):

For reference, this MD5 sum is calculated from the MD5 text of the .msg file, where the MD5 text is the .msg text with:

• comments removed
• whitespace removed
• package names of dependencies removed
• constants reordered ahead of other declarations

In order to catch changes that occur in embedded message types, the MD5 text is concatenated with the MD5 text of each of the embedded types, in the order that they appear.

So, provided that the type never changes, and message contents or values are not considered: no, the MD5 sum never changes.