Inject content without removing previous content - Append/prepend option

Hi,
I need to inject scripts and pieces of code in existing code. Is there a way to add content inside placeholders without removing those entered previously? Like append/prepend option.
Thank you

Start by locating the gulp-inject entry point and the code that handles existing placeholders, then check whether the repository has tests covering repeated injection. Define done as supporting append and prepend behavior without removing previously injected content, with tests for both options.

Written by the indexing model from the issue text.