If they are the same amino acids in the same order, you would not see anything, since the CD spectra are mirror images with respect to the wavelength axis. There are a few papers claiming that a fundamental asymmetry in nature (parity violation of the electro-weak nuclear force) can have chemical effects and may be responsible for the homochirality we observe in proteins I find that rather (well, highly, actually) doubtful.

In the attached paper (which is about the above effect, maybe you like the idea), CD spectra are shown related to your question in figure 1, which shows that if you add them up (as in a 50/50 mixture) there would be nothing left. Maybe the section Residual CD signal in poly- DL amino acids (p. 337) is of interest to you.

so there may be a difference in the purity or enantiomeric excess of the AAs used to make the peptide. 

Also depending upon how big peptides are, they may be folded differently. This would require a chiral environment when they folded which would depend upon how you made them.

Also CD from peptides can be strong, maybe you don't have exactly a 50/50 mixture. 

In my estimation, D- and L- peptides, if you are talking about the same amino acid sequence, should give you mirror images when comparing.

See this reference for more

https://www.ncbi.nlm.nih.gov/pubmed/21903489

Here, they have single amino acids though, not a sequence.

Best,

 Sezgin

Thanks all for the valuable suggestion! and yes my query was about peptide of same amino acids. 

This is a question which has an unexpected answer. It is possible to "shift" this discussion by asking what is observed one mixes one mole of an L-isomer and one mole of a DL isomer. The question is then simply whether LL pair and a DL pair are identical.  The question then is whether spectra are identical to only one L isomer present or something else.  In the solid state in fact vibrational spectroscopy and Solid State NMR shows a  3:1 mixture is not at all the same as a 1:0 mixture.   Have two publications on this on LLDL amino acids.  

am adding the two references:

13C CP/MAS of LDLL mixtures of amino acids  Solid State Nuclear Magnetic Resonance  2(1993) 11-20.

Vibrational spectroscopy of LDLL-mixtures of amino acids 6(1994) 293-299.

LL- affinity can be less than,  greater than or equal to DL affinity.

The spectroscopic evidence is [in the solid state], the LDLL spectra were very different than either the L-isomer or the DL-isomer.

Whether the same is true at the interface of solutions and solids or in the solution state,  I am not certain  / have no experimental evidence.

Seems to me one cannot actually properly characterize the properties of amino acids without including solids properties.

Have recently published an article on the vibrational spectroscopy of two dipeptides Ala-Pro and Pro-Ala in the solid state.

Continous gradient temperature Raman Spectroscopy identifies flexible sites in proline and alanine peptides  Vibrational Spectroscopy 80(2015) 59-65.

Totally unambiguous  that Ala-Pro has the same physical properties as Pro-Ala.

AND 3:1 mixtures in the solid state are not at all the same as either Ala-Pro or Pro-Ala.

Walter Schmidt and Gert Van der Zwan and others: on a somewhat similar topic, if I have a sample of D and L free amino acids, can CD be used to determine the ratio of L to D enantiomers? I am not familiar with CD!

CD is circular dichroism.  In essence,  one isomer give a sine function whereas the other gives a -sine function.   When both are present in equal amounts, there is no net signal.  Instrumentation depends upon whether light is polarized towards the left or towards the right as a function of wavelength. 

My recommendation is that if you want to separate D from L isomer, please investigate chromatography with a chiral stationary phase.

Thank you for the quick reply Walter!