2 Words
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PowerShell, 120 bytes

param($s,$c)($s,(($l=$s.length-1)..0+1..$l|%{($x=$c*(2*$_))+($s[($_,($l..$_+($_+1)..$l))[$_-ne$l]]-join$c)+$x}))[$l-gt0]

Try it online!

Some days, having index ranges instead of slices really hurts. Today is one of those days. Due to conjoined ranges messing up when dealing with single elements (e.g. returning 0..0+1..0), special-casing is used to avoid it altogether (at the cost of many bytes).

PowerShell, 120 bytes

param($s,$c)($s,(($l=$s.length-1)..0+1..$l|%{($x=$c*(2*$_))+($s[($_,($l..$_+($_+1)..$l))[$_-ne$l]]-join$c)+$x}))[$l-gt0]

Try it online!

Some days, having index ranges instead of slices really hurts. Today is one of those days.

PowerShell, 120 bytes

param($s,$c)($s,(($l=$s.length-1)..0+1..$l|%{($x=$c*(2*$_))+($s[($_,($l..$_+($_+1)..$l))[$_-ne$l]]-join$c)+$x}))[$l-gt0]

Try it online!

Some days, having index ranges instead of slices really hurts. Today is one of those days. Due to conjoined ranges messing up when dealing with single elements (e.g. returning 0..0+1..0), special-casing is used to avoid it altogether (at the cost of many bytes).

1
source | link

PowerShell, 120 bytes

param($s,$c)($s,(($l=$s.length-1)..0+1..$l|%{($x=$c*(2*$_))+($s[($_,($l..$_+($_+1)..$l))[$_-ne$l]]-join$c)+$x}))[$l-gt0]

Try it online!

Some days, having index ranges instead of slices really hurts. Today is one of those days.