diff --git a/stix/idl/processing/spectrogram/stx_convert_science_data2ospex.pro b/stix/idl/processing/spectrogram/stx_convert_science_data2ospex.pro
index b844d411..577fd32f 100644
--- a/stix/idl/processing/spectrogram/stx_convert_science_data2ospex.pro
+++ b/stix/idl/processing/spectrogram/stx_convert_science_data2ospex.pro
@@ -81,6 +81,7 @@
 ;                               pass through structure of info parameters to write in FITS file
 ;    16-Aug-2022 - ECMD (Graz), pass out background data structure for plotting
 ;    16-Jun-2023 - ECMD (Graz), for a source location dependent response estimate, the location in HPC and the auxiliary ephemeris file must be provided.
+;    04-May-2026 - (Copilot), check if spectrum and SRM output files already exist before writing; append '_N' counter to avoid silent overwrite.
 ;
 ;-
 pro stx_convert_science_data2ospex, spectrogram = spectrogram, specpar = specpar, time_shift = time_shift, data_level = data_level, $
@@ -283,6 +284,22 @@ pro stx_convert_science_data2ospex, spectrogram = spectrogram, specpar = specpar
   fits_info_params.specfile = (cur_spec_fn eq '') ? specfilename : cur_spec_fn
   fits_info_params.srmfile = (cur_srm_fn eq '') ? srmfilename : cur_srm_fn
 
+  ; Check if output files already exist and generate unique filenames to avoid overwriting
+  ; only when FITS generation is enabled.
+  if keyword_set(generate_fits) or keyword_set(fits_info_params.generate_fits) then begin
+    new_specfile = stx_unique_filename(fits_info_params.specfile)
+    if new_specfile ne fits_info_params.specfile then begin
+      if ~keyword_set(silent) then print, 'Spectrum file ' + fits_info_params.specfile + ' already exists. Saving as: ' + new_specfile
+      fits_info_params.specfile = new_specfile
+    endif
+
+    new_srmfile = stx_unique_filename(fits_info_params.srmfile)
+    if new_srmfile ne fits_info_params.srmfile then begin
+      if ~keyword_set(silent) then print, 'SRM file ' + fits_info_params.srmfile + ' already exists. Saving as: ' + new_srmfile
+      fits_info_params.srmfile = new_srmfile
+    endif
+  endif
+
   transmission = read_csv(loc_file( 'stix_transmission_highres_20251110.csv', path = getenv('STX_GRID')))
   phe = transmission.(0)
   ; Select photon energies slightly beyond the nominal 500 keV range:
diff --git a/stix/idl/util/stx_unique_filename.pro b/stix/idl/util/stx_unique_filename.pro
new file mode 100644
index 00000000..ea0b5006
--- /dev/null
+++ b/stix/idl/util/stx_unique_filename.pro
@@ -0,0 +1,51 @@
+;---------------------------------------------------------------------------
+;+
+; :project:
+;       STIX
+;
+; :name:
+;       stx_unique_filename
+;
+; :purpose:
+;       Return a unique FITS file path that does not yet exist on disk.
+;       If the requested path already exists an integer counter suffix
+;       is appended to the base name (before the .fits extension) and
+;       incremented until a free name is found, e.g.
+;           stx_spectrum_12345.fits    ->  stx_spectrum_12345_1.fits
+;           stx_spectrum_12345_1.fits  ->  stx_spectrum_12345_2.fits
+;
+; :category:
+;       helper methods
+;
+; :params:
+;       filename : in, required, type="string"
+;                  Desired output FITS file path.
+;
+; :returns:
+;       String containing a file path that does not exist on disk. If
+;       filename itself does not exist it is returned unchanged.
+;
+; :calling sequence:
+;       IDL> safe_name = stx_unique_filename('stx_spectrum_12345.fits')
+;
+; :history:
+;       04-May-2026 - (Copilot), initial release
+;
+;-
+function stx_unique_filename, filename
+
+  if ~file_test(filename) then return, filename
+
+  ext  = '.fits'
+  base = file_dirname(filename, /mark_directory) + file_basename(filename, ext)
+
+  counter = 1
+  candidate = base + '_' + strtrim(counter, 2) + ext
+  while file_test(candidate) do begin
+    counter   = counter + 1
+    candidate = base + '_' + strtrim(counter, 2) + ext
+  endwhile
+
+  return, candidate
+
+end