Many students have read about topic sentences and believe that it's essential to make a separate sentence to introduce the topic before making any kind of argumentative claim:
(sample body paragraph #1)
The first issue to discuss relates to the use of police time in the enforcement of marijuana laws. If marijuana is legalised then valuable police time will be saved. First of all, supporting idea one blah blah blah blah blah. In addition, supporting idea two blah blah blah blah blah. Furthermore, supporting idea three blah blah blah blah blah. Finally, supporting idea four blah blah blah blah blah.
This is unnecessary repetition. The second sentence already contains the topic AND the main claim of the paragraph, and so it already behaves like a topic sentence:
(sample body paragraph #2)
If marijuana is legalised then valuable police time will be saved. First of all, supporting idea one blah blah blah blah blah. In addition, supporting idea two blah blah blah blah blah. Furthermore, supporting idea three blah blah blah blah blah. Finally, supporting idea four blah blah blah blah.
In academic writing it is generally a good idea to get to the pointas quickly and concisely as possible. This kind of writing is much easier to read and is more likely to result in a good score in IELTS Task 2 for task response and coherence and cohesion.